pm: Optimize pre-wakeup in suspend procedure

When core is suspended and power mode has non zero exit_latency_us
a system timeout will be rescheduled to a point in time that is
earlier by exit_latency_us than request. It is to accommodate for
lengthy resuming procedure which would cause requested timeout to
be significantly late. However, setting additional wake up point
has cost, it is one more redundant core wake up and that impacts
performance and power consumption.

Add Kconfig option to chose what conversion method is used. It has
the biggest impact on small exit_latency_us where conversion may
result in 0 ticks (no pre-wake up) or 1 tick (wake up).

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