doc: cmd: add documentation for cpu command

Add documentation for the 'cpu' command, taking NXP i.MX 8M Plus
as a example.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 49b354e..1f6518b 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -50,6 +50,7 @@
    cmd/coninfo
    cmd/conitrace
    cmd/cp
+   cmd/cpu
    cmd/cyclic
    cmd/dm
    cmd/ebtupdate