arm/dt: dory: add low fps mode

add low-fps-mode for ambient mode to able to control FRAMERATE_CTRL.
on ambient mode(prepare), it will set 50Hz to reduce the power consumption,
on resume(complete), it will restore framerate as 60Hz

Change-Id: I70490192aa356dd1ca0cc9c0b0fa76bd670c3480
Signed-off-by: Se Hun Kim <sean2.kim@lge.com>