pm: policy: allow multiple subscribers to latency changes

Current API allowed to get notified when the maximum system latency
changes, however, a single callback was allowed. The design was intended
for SoC specific actions when latency changes. However, in some cases
drivers may also want to know the current maximum latency to perform
local actions if other parts of the system modify it.

This patch updates the API with a pair of subscribe/unsubscribe calls to
achieve such goal. Tests have been updated to show how the API can be
used.

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