powercap: arm_scmi: Add SCMI Powercap based driver

Add a powercap driver that, using the ARM SCMI Protocol to query the SCMI
platform firmware for the list of existing Powercap domains, registers all
of such discovered domains under the new 'arm-scmi' powercap control type.

A new simple powercap zone and constraint is registered for all the SCMI
powercap zones that are found.

Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>