doc: cmd: add documentation for scmi

This is a help text for scmi command.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index d8e23fc..1a626c0 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -94,6 +94,7 @@
    cmd/rng
    cmd/saves
    cmd/sbi
+   cmd/scmi
    cmd/scp03
    cmd/seama
    cmd/setexpr