MAINTAINERS: add MT2712 RTC files

This patch add MT2712 RTC related files to MAINTAINERS file.

Signed-off-by: Ran Bi <ran.bi@mediatek.com>
Link: https://lore.kernel.org/r/20200226051303.22560-5-ran.bi@mediatek.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>