PM / devfreq: rockchip_dmc: change frequency according to system status

PM / devfreq: rockchip_dmc: change frequency according to system status

This registers a reboot notifier, it will change ddr frequency to a
specified value when reboot system.

This registers a fb notifier, it will change ddr frequency to a specified
value and enable ddr self-reflash mode when early suspend.

This adds a new sysfs node system_status, so that different system status
can change ddr frequency through the node.

Change-Id: Ib5d7d5bd8ee82c29f6f260a3d2ffcb829dde2003
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>