dt-bindings: rtc: Move trivial RTC over to a schemas of their own

The RTC generic bindings has a bunch of devices that have a pretty simple
binding, with just compatible, reg and optional interrupts properties.

This is exactly what the trivial devices YAML schema has been created for,
except that they can also have the start-year property, but not any other
generic RTC property.

Let's create a schema with those constraints.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>