binman: Add support for SCP firmware

Add an entry type for a firmware blob for a System Control Processor,
given by an entry arg. This firmware is a raw binary blob.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>