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>