dt-bindings: rtc: stmp3xxx-rtc: convert to dtschema

Convert existing binding to dtschema to support validation and
add the undocumented compatible 'fsl,imx23-rtc'.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240413-rtc_dtschema-v3-4-eff368bcc471@gmail.com


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