PM / devfreq: rk3399_dmc: Support new disable-freq properties

Implement the newly-defined properties to allow disabling certain
power-saving-at-idle features at higher frequencies.

This is a rewritten version of work by Lin Huang <hl@rock-chips.com>.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>