power-internal: Add getter and signal subscriber for lock count

power-internal: Add getter and signal subscriber for lock count

int device_power_get_lock_count()
 : Get the current lock count that hasn't yet been unlocked.

int device_power_add_lock_count_change_callback()
 : Register callback function that is invoked on lock count changed.
   The lock count is increased by 1 on device_power_request_lock(),
   decreased by 1 on device_power_release_lock() or timeout.

int device_power_remove_lock_count_change_callback()
 : Unregister the callback registered by the above.

Change-Id: Ib77260c867a27013572ee42ab1264ad3c3e5794e
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>