rtc: mt6397: implement suspend/resume function in rtc-mt6397 driver

Implement the suspend/resume function in order to control rtc's irq_wake flag and handle as wakeup source.

Signed-off-by: Henry Chen <henryc.chen@mediatek.com>
Acked-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>