firmware: arm_scmi: Refactor the internal powercap get/set helpers

Refactor the SCMI powercap internal get/set helpers in preparation to
adding the powercap protocol enable/disable support.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20230531152039.2363181-2-cristian.marussi@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>