Documentation: ACPI: move osi.txt to firmware-guide/acpi and convert to reST

This converts the plain text documentation to reStructuredText format
and adds it to Sphinx TOC tree.

No essential content change.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/firmware-guide/acpi/index.rst

@@ -9,3 +9,4 @@ ACPI Support
 
    namespace
    enumeration
+   osi

Documentation/acpi/osi.txt → Documentation/firmware-guide/acpi/osi.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
 ACPI _OSI and _REV methods
---------------------------
+==========================
 
 An ACPI BIOS can use the "Operating System Interfaces" method (_OSI)
 to find out what the operating system supports. Eg. If BIOS
@@ -14,7 +17,7 @@ This document explains how and why the BIOS and Linux should use these methods.
 It also explains how and why they are widely misused.
 
 How to use _OSI
----------------
+===============
 
 Linux runs on two groups of machines -- those that are tested by the OEM
 to be compatible with Linux, and those that were never tested with Linux,
@@ -62,7 +65,7 @@ the string when that support is added to the kernel.
 That was easy.  Read on, to find out how to do it wrong.
 
 Before _OSI, there was _OS
---------------------------
+==========================
 
 ACPI 1.0 specified "_OS" as an
 "object that evaluates to a string that identifies the operating system."
@@ -96,7 +99,7 @@ That is the *only* viable strategy, as that is what modern Windows does,
 and so doing otherwise could steer the BIOS down an untested path.
 
 _OSI is born, and immediately misused
---------------------------------------
+=====================================
 
 With _OSI, the *BIOS* provides the string describing an interface,
 and asks the OS: "YES/NO, are you compatible with this interface?"
@@ -144,7 +147,7 @@ catastrophic failure resulting from the BIOS taking paths that
 were never validated under *any* OS.
 
 Do not use _REV
----------------
+===============
 
 Since _OSI("Linux") went away, some BIOS writers used _REV
 to support Linux and Windows differences in the same BIOS.
@@ -164,7 +167,7 @@ from mid-2015 onward.  The ACPI specification will also be updated
 to reflect that _REV is deprecated, and always returns 2.
 
 Apple Mac and _OSI("Darwin")
-----------------------------
+============================
 
 On Apple's Mac platforms, the ACPI BIOS invokes _OSI("Darwin")
 to determine if the machine is running Apple OSX.