dt-bindings: rtc: pxa-rtc: convert to dtschema

Convert existing binding to dtschema to support validation.

The missing 'reg' and 'interrupts' properties have been added, taking
the 2 supported interrupts into account to fix the example.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://lore.kernel.org/r/20240413-rtc_dtschema-v3-3-eff368bcc471@gmail.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>