rtc: mxc: use of_device_get_match_data

Use of_device_get_match_data to simplify mxc_rtc_probe.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20201117203035.1280099-1-alexandre.belloni@bootlin.com