firmware: arm_scpi: Add hardware dependencies

With a name like that, I assume that the ARM SCPI protocol is only
useful on the ARM architectures.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Cc: Jon Medhurst (Tixy) <tixy@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>