rtc: mediatek: update MAINTAINERS entry with MediaTek RTC driver

I work for MediaTek on maintaining the MediaTek SoC based RTC driver for
the existing SoCs and keep adding support for the following SoCs in the
future.

Cc: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>