PM / Domains: Don't measure latency of ->power_on|off() during system PM

Measure latency does by itself contribute to an increased latency, thus we
should avoid it when it isn't needed.

Currently genpd measures latencies in the system PM phase for the
->power_on|off() callbacks, except in the syscore case when it's not
allowed to use ktime_get() as timekeeping may be suspended.

Since there should be plenty of occasions during runtime PM to perform
these measurements, let's rely on that and drop them from system PM. This
will also make it consistent for how measurements are done of the runtime
PM callbacks (as those may be invoked during system PM).

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>