docs: dt: Add DT API documentation

The kernel-doc for the DT APIs are not included in the documentation
build. Add them.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-9-robh@kernel.org

Documentation/devicetree/index.rst

@@ -11,6 +11,7 @@ Kernel Devicetree Usage
 
    usage-model
    of_unittest
+   kernel-api
 
 Devicetree Overlays
 ===================

Documentation/devicetree/kernel-api.rst

+.. SPDX-License-Identifier: GPL-2.0
+.. _devicetree:
+
+======================================
+DeviceTree Kernel API
+======================================
+
+Core functions
+--------------
+
+.. kernel-doc:: drivers/of/base.c
+   :export:
+
+.. kernel-doc:: include/linux/of.h
+   :internal:
+
+.. kernel-doc:: drivers/of/property.c
+   :export:
+
+.. kernel-doc:: include/linux/of_graph.h
+   :internal:
+
+.. kernel-doc:: drivers/of/address.c
+   :export:
+
+.. kernel-doc:: drivers/of/irq.c
+   :export:
+
+.. kernel-doc:: drivers/of/fdt.c
+   :export:
+
+Driver model functions
+----------------------
+
+.. kernel-doc:: include/linux/of_device.h
+   :internal:
+
+.. kernel-doc:: drivers/of/device.c
+   :export:
+
+.. kernel-doc:: include/linux/of_platform.h
+   :internal:
+
+.. kernel-doc:: drivers/of/platform.c
+   :export:
+
+Overlay and Dynamic DT functions
+--------------------------------
+
+.. kernel-doc:: drivers/of/resolver.c
+   :export:
+
+.. kernel-doc:: drivers/of/dynamic.c
+   :export:
+
+.. kernel-doc:: drivers/of/overlay.c
+   :export: