test: dm: add scmi command test

In this test, "scmi" command is tested against different sub-commands.
Please note that scmi command is for debug purpose and is not intended
in production system.

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>