PM / Domains: Extend generic power domain debugfs

This patch extends the existing generic power domain debugfs.
Changes involve the following
- Introduce a unique debugfs entry for each generic power domain with the
  following attributes
	- current_state - Displays current state of the domain.
	- devices - Displays the devices associated with this domain.
	- sub_domains - Displays the sub power domains.
	- active_time - Displays the time the domain was in active state
			in ms.
	- total_idle_time - Displays the time the domain was in any of the idle
			states in ms.
	- idle_states - Displays the various idle states and the time
			spent in each idle state in ms.

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