PM: domains: Move genpd's time-accounting to ktime_get_mono_fast_ns()

To move towards a more consistent behaviour between genpd and the runtime
PM core, let's start by converting genpd's time-accounting from ktime_get()
into ktime_get_mono_fast_ns().

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