dt-bindings: rtc: convert MT7622 RTC to the json-schema

This helps validating DTS files. Introduced changes:
1. Reworded title
2. Dropper redundant properties descriptions
3. Added required #include-s and adjusted "reg" in example

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240123125043.27192-1-zajec5@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>