PM / devfreq: imx8m-ddrc: Remove unneeded of_match_ptr()

i.MX is a DT-only platform, so of_match_ptr() can be safely
removed.

Remove the unneeded of_match_ptr().

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>