pm: policy: keep microseconds resolution

When new latency requirements are introduced/updated, the microseconds
value gets converted down to ticks. Ticks usually have a coarse
resolution compared to the microseconds scale. This is fine for making
PM state change decisions, however, when getting notified about latency
changes, we may want to know the real value in microseconds, even if the
system rounds to ticks internally. This patch stores the value in both
us and ticks (in ticks to cache the conversion, really), so that the
user will get notified with precise latency values.

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