dt-bindings: rtc: microcrystal,rv3032: Document wakeup-source property

The RV3032 can be used as a wake-up source, document the 'wakeup-source'
property as allowed property.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230921192604.70944-1-marex@denx.de


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