PM-runtime: Add new interface to get accounted time

Some drivers (like i915/drm) needs to get the accounted suspended time.
pm_runtime_suspended_time() will return the suspended accounted time
in ns unit.

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