powercap: add suspend and resume mechanism for SOC power limit

PL1 and PL2 could be throlled or de-throttled by
Thermal management to control SOC temperature.

However, currently, their value will be reset to default value
after once system suspend and resume.

Add pm_notifier to save PL1, PL2 before system suspect and restore
PL1, PL2 after system resume.

Signed-off-by: Zhen Han <zhen.han@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>