PM / devfreq: rk3399_dmc: Support new *-ns properties

We want to keep the idle time fixed, so compute based on the current DDR
frequency.

The old properties were deprecated and never used, so we can safely drop
them from the driver.

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>