PM / devfreq: imx8m-ddrc: Remove DEVFREQ_GOV_SIMPLE_ONDEMAND dependency

The driver can't support simple ondemand governor due to missing
.get_dev_status() capability.

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