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>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 8b98629..16a3db5 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -43,6 +43,7 @@
    cmd/false
    cmd/fatinfo
    cmd/fatload
+   cmd/fdt
    cmd/for
    cmd/load
    cmd/loadm