mod/power_capping_hal: Add power notifications and threshold support

Implement support in the Power capping HAL for sending notifications:
 - When the power averaging interval (PAI) changes.
 - When the average power consumption of the power capping domain,
   measured over PAI, is outside the threshold values set by the agent
   for the power capping domain.

Also add support for setting low/high power thresholds using
mod_pcapping_set_power_thresholds(), which allows agents to
define when measurement-based notifications should occur
for the power capping domain.

Update scmi_power_capping to allow agents to subscribe to
and process these new notifications.

Signed-off-by: Mahmoud Elsabbagh <mahmoud.elsabbagh@arm.com>