pm: Add option to quickly detect power state availability

When all states are locked or latency requirement cannot be met
by any power state it is important to be able to quickly exit
suspend procedure because that usually means that application
requires high performance. Add function for detecting if any
power state is available.

Additionally, add function pm_policy_state_is_available for
checking if given state is available which means that it is not
locked and fulfills current latency requirement.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>