fdt: add fdt description document

Signed-off-by: liaoao <liaoao@xiaomi.com>

Documentation/guides/devicetree.rst

@@ -0,0 +1,36 @@
+====================
+Device Tree
+====================
+
+Overview
+--------
+
+Currently, NuttX supports parsing of FDT(Flattened Device Tree) using libfdt, a
+utility library for reading and manipulating the binary format:
+
+https://github.com/dgibson/dtc/
+
+Based on that, NuttX has implemented some common functions to get properties.
+Device Tree support in NuttX will reduce the configuration of chips/boards,
+it is not used in NuttX kernel framework yet.
+
+How to use
+-----------
+
+1. Enable Device Tree and libfdt
+
+Enable the Kconfig settings:
+
+    .. code-block:: console
+
+      CONFIG_DEVICE_TREE=y                        /* Enable Device Tree */
+      CONFIG_LIBFDT=y                             /* Enable utility library */
+
+2. Register the DTB address
+
+Use fdt_register to set the DTB Address in NuttX
+
+3. Parse the DTB
+
+Chip/board will use fdt_get to get the DTB Address, and then use fdt_* APIs to
+parse the DTB properties.

Documentation/guides/index.rst

@@ -34,3 +34,4 @@ Guides
   stackrecord.rst
   etcromfs.rst
   thread_local_storage.rst
+  devicetree.rst