PM / devfreq: event: support rockchip dfi controller

on rk3399 platform, there is dfi conroller can monitor
ddr load, base on this result, we can do ddr freqency
scaling.

Signed-off-by: Lin Huang <hl@rock-chips.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>