PM / devfreq: rk3399_dmc: Drop undocumented ondemand DT props

These properties are:

* undocumented
* directly representing software properties, not hardware properties
* unused (no in-tree users, yet; this IP block has so far only been used
  in downstream kernels)

Let's just stick the values that downstream users have been using
directly in the driver and call it a day.

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