PM / devfreq: rockchip: add devfreq driver for rk3399 dmc

base on dfi result, we do ddr frequency scaling, register
dmc driver to devfreq framework, and use simple-ondemand
policy.

Signed-off-by: Lin Huang <hl@rock-chips.com>
Signed-off-by: MyngJoo Ham <myngjoo.ham@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>