pm: policy: fix pm_policy_event_register arg

The pm_policy_event_register() API takes absolute cycles as the
second arg, like pm_policy_event_update(), but the arg is renamed
time_us and treated as a relative time in us rather than abs
cycles.

Fix implementation of pm_policy_event_register() to treat cycles
like pm_policy_event_update() and API docs suggest.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>