doc: Make a start on docs for fdt command

Add some information about the 'fdt addr' subcommand, to get things
started.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/fdt.rst

@@ -0,0 +1,69 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+fdt command
+===========
+
+Synopis
+-------
+
+::
+
+    fdt addr [-cq] [addr [len]]
+
+Description
+-----------
+
+The fdt command provides access to flat device tree blobs in memory. It has
+many subcommands, some of which are not documented here.
+
+Flags:
+
+-c
+    Select the control FDT (otherwise the working FDT is used).
+-q
+    Don't display errors
+
+The control FDT is the one used by U-Boot itself to control various features,
+including driver model. This should only be changed if you really know what you
+are doing, since once U-Boot starts it maintains pointers into the FDT from the
+various driver model data structures.
+
+The working FDT is the one passed to the Operating System when booting. This
+can be freely modified, so far as U-Boot is concerned, since it does not affect
+U-Boot's operation.
+
+fdt addr
+~~~~~~~~
+
+With no arguments, this shows the address of the current working or control
+FDT.
+
+If the `addr` argument is provided, then this sets the address of the working or
+control FDT to the provided address.
+
+If the `len` argument is provided, then the device tree is expanded to that
+size. This can be used to make space for more nodes and properties. It is
+assumed that there is enough space in memory for this expansion.
+
+Example
+-------
+
+Get the control address and copy that FDT to free memory::
+
+    => fdt addr -c
+    Control fdt: 0aff9fd0
+    => cp.b 0aff9fd0 10000 10000
+    => md 10000 4
+    00010000: edfe0dd0 5b3d0000 78000000 7c270000  ......=[...x..'|
+
+The second word shows the size of the FDT. Now set the working FDT to that
+address and expand it to 0xf000 in size::
+
+    => fdt addr 10000 f000
+    => md 10000 4
+    00010000: edfe0dd0 00f00000 78000000 7c270000  ...........x..'|
+
+Return value
+------------
+
+The return value $? indicates whether the command succeeded.

doc/usage/index.rst

@@ -43,6 +43,7 @@ Shell commands
    cmd/false
    cmd/fatinfo
    cmd/fatload
+   cmd/fdt
    cmd/for
    cmd/load
    cmd/loadm