cmd: add scmi command for SCMI firmware

This command, "scmi", may provide a command line interface to various SCMI
protocols. It supports at least initially SCMI base protocol and is
intended mainly for debug purpose.

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>