module/scmi_power_capping: restructure

Create a common core file that can be used by fast channels and
normal SCMI Power Capping protocol commands.

The common core file will handle the interactions with power
capping HAL and handle SCMI agents' requests synchronisation
between normal commands and fast channels.

The change would simplify the implementation and make the
module easier to extend.

Changes to fast channels are done in the next patch.

Signed-off-by: Ahmed Gadallah <ahmed.gadallah@arm.com>