rtc: mxc: Convert the driver to DT-only

Since 5.10-rc1 i.MX is a devicetree-only platform, so simplify the code
by removing the unused non-DT support.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201116180326.5199-1-festevam@gmail.com