Implement power management support.

Original implementation is from Zhao, Forrest <forrest.zhao@intel.com>
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NZhao, Forrest <forrest.zhao@intel.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>