dt-bindings: rtc: Convert apm,xgene-rtc to DT schema

Convert the APM XGene RTC binding to DT schema format. It's a
straight-forward conversion.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250924222848.2949235-1-robh@kernel.org


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