doc: describe the scp03 command

The Secure Channel Protocol 03 command sends control requests
(enable/provision) to the TEE implementing the protocol between the
processor and the secure element.

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index f7b706f..7fac2e4 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -33,3 +33,4 @@
    qfw
    sbi
    true
+   scp03