rtc: mt6397: refine RTC_TC_MTH

This patch adds RTC_TC_MTH_MASK to support new chips.

Signed-off-by: Yuchen Huang <yuchen.huang@mediatek.com>
Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>