firmware: arm_scmi: Enable building as a single module

Now, with all the plumbing in place to enable building scmi as a module
instead of built-in modules, let us enable the same.

Link: https://lore.kernel.org/r/20200907195046.56615-5-sudeep.holla@arm.com


Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>