PM / devfreq: imx8m-ddrc: Remove imx8m_ddrc_get_dev_status

Current driver actually does not support simple ondemand governor
as it's unable to provide device load information. So removing
the unnecessary callback to avoid confusing.
Right now the driver is using userspace governor by default.

polling_ms was also dropped as it's not needed for non-ondemand
governor.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>