dt-bindings: rtc: s3c: Include generic dt-schema bindings

Include the generic rtc.yaml bindings in Samsung S3C RTC bindings.  This
brings the requirement of proper node names and adds parsing of
additional properties.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>