dt-bindings: rtc: nuvoton: add NCT3018Y Real Time Clock

Document devicetree bindings for the Nuvoton NCT3018Y Real Time Clock.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mia Lin <mimi05633@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220713090647.8028-2-mimi05633@gmail.com