docs: Add documentation for ARC processors

ARC processors are supported in upstream kernel since v3.9
and so far there was no documentation about them except some
Device Tree bindings.

Fixing it with the simples set of docs now:
1. Overview with pointers to other informational resources
2. Autogenerated feature table

Note though it's just the very beginning, there will be more
for sure given time as there're many things worth documenting
and in fact even contents itself is avaialble but just spread
in some other places. Now we'll try to keep all here and
then maintain it looking forward to match the state of development.
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Vineet Gupta <vgupta@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20211112065059.7273-1-abrodkin@synopsys.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arc/arc.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+Linux kernel for ARC processors
+*******************************
+
+Other sources of information
+############################
+
+Below are some resources where more information can be found on
+ARC processors and relevant open source projects.
+
+1. `<https://embarc.org/>`_ - Community portal for open source on ARC.
+Good place to start to find relevant FOSS projects, toolchain releases,
+news items and more.
+
+2. `<https://github.com/foss-for-synopsys-dwc-arc-processors>`_ -
+Home for all development activities regarding open source projects for
+ARC processors. Some of the projects are forks of various upstream projects,
+where "work in progress" is hosted prior to submission to upstream projects.
+Other projects are developed by Synopsys and made available to community
+as open source for use on ARC Processors.
+
+3. `<https://www.synopsys.com/designware-ip/processor-solutions.html>`_ -
+Official Synopsys ARC Processors website location, with access to some IP
+documentation (Programmer's Reference Manuals, AKA "PRM's", see
+`<https://www.synopsys.com/dw/doc.php/ds/cc/programmers-reference-manual-ARC-HS.pdf>`_)
+and commercial tools (Free nSIM,
+`<https://www.synopsys.com/cgi-bin/dwarcnsim/req1.cgi>`_ and
+MetaWare Light Edition, `<https://www.synopsys.com/cgi-bin/arcmwtk_lite/reg1.cgi>`_)
+
+Important note on ARC processors configurability
+################################################
+
+ARC processors are highly configurable and several configurable options
+are supported in Linux. Some options are transparent to software
+(i.e cache geometries, some can be detected at runtime and configured
+and used accordingly, while some need to be explicitly selected or configured
+in the kernel's configuration utility (AKA "make menuconfig").
+
+However not all configurable options are supported when an ARC processor
+is to run Linux. SoC design teams should refer to "Appendix E:
+Configuration for ARC Linux" in the ARC HS Databook for configurability
+guidelines.
+
+Following these guidelines and selecting valid configuration options
+up front is critical to help prevent any unwanted issues during
+SoC bringup and software development in general.
+
+Building the Linux kernel for ARC processors
+############################################
+
+The process of kernel building for ARC processors is the same as for any other
+architecture and could be done in 2 ways:
+
+1. cross-compilation: process of compiling for ARC targets on a development
+host with a different processor architecture (generally x86_64/amd64).
+
+2. native compilation: process of compiling for ARC on a ARC platform
+(hardware board or a simulator like QEMU) with complete development environment
+(GNU toolchain, dtc, make etc) installed on the platform.
+
+In both cases, up-to-date GNU toolchain for ARC for the host is needed.
+Synopsys offers prebuilt toolchain releases which can be used for this purpose,
+available from:
+
+1. Synopsys GNU toolchain releases:
+`<https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/releases>`_
+2. Linux kernel compilers collection:
+`<https://mirrors.edge.kernel.org/pub/tools/crosstool/>`_
+3. Bootlin's toolchain collection: `<https://toolchains.bootlin.com/>`_
+
+Once the toolchain is installed in the system, make sure its "bin" folder
+is added in your ``PATH`` environment variable. Then set ``ARCH=arc`` &
+``CROSS_COMPILE=arc-linux`` (or whatever matches installed ARC toolchain prefix)
+and then as usual ``make defconfig && make``.
+
+This will produce "vmlinux" file in the root of the kernel source tree
+usable for loading on the target system via JTAG.
+If you need to get an image usable with U-Boot bootloader,
+type ``make uImage`` and ``uImage`` will be produced in ``arch/arc/boot``
+folder.

Documentation/arc/features.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. kernel-feat:: $srctree/Documentation/features arc

Documentation/arc/index.rst

+===================
+ARC architecture
+===================
+
+.. toctree::
+    :maxdepth: 1
+
+    arc
+
+    features
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`

Documentation/arch.rst

@@ -9,6 +9,7 @@ implementation.
 .. toctree::
    :maxdepth: 2
 
+   arc/index
    arm/index
    arm64/index
    ia64/index

MAINTAINERS

@@ -18291,6 +18291,7 @@ M:	Vineet Gupta <vgupta@kernel.org>
 L:	linux-snps-arc@lists.infradead.org
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/vgupta/arc.git
+F:	Documentation/arc/
 F:	Documentation/devicetree/bindings/arc/*
 F:	Documentation/devicetree/bindings/interrupt-controller/snps,arc*
 F:	arch/arc/