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

add missing devicetree-binding document for mt6397 rtc
in later patch driver is extended with mt6323 chip
Suggested-By: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Josef Friedl <josef.friedl@speed.at>
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/rtc/rtc-mt6397.txt

+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";
+               };
+       };