dt-bindings: rtc: mediatek: add missing mt6397 rtc

Device-Tree bindings for MediaTek PMIC based RTC

MediaTek PMIC based RTC is an independent function of MediaTek PMIC that works

as a type of multi-function device (MFD). The RTC can be configured and set up

with PMIC wrapper bus which is a common resource shared with the other

functions found on the same PMIC.

For MediaTek PMIC MFD bindings, see:

../mfd/mt6397.txt

For MediaTek PMIC wrapper bus bindings, see:

../soc/mediatek/pwrap.txt

Required properties:

- compatible: Should be one of follows

       "mediatek,mt6323-rtc": for MT6323 PMIC

       "mediatek,mt6397-rtc": for MT6397 PMIC

Example:

       pmic {

               compatible = "mediatek,mt6323";

               ...

               rtc {

                       compatible = "mediatek,mt6323-rtc";

               };

       };