PM: sleep: call devfreq suspend/resume

Devfreq framework supports suspend of its devices.
Call the the devfreq interface and allow devfreq devices
preserve/restore their states during suspend/resume.

Suggested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>