rtc: mxc: 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: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210315235800.200137-1-festevam@gmail.com