pm: policy: new API to set maximum latency requirements

Add a new API that allows to configure maximum latency requirements.
When the policy manager computes the next state, it will check if the
state brings too much latency based on requirements. This can be useful,
for example, if a certain driver or the application want a system to
respond fast, since any low power state that brings too much latency
will not be used.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>