rtc: m41t80: Use the unified property API get the wakeup-source property

This allows both ACPI and Device Tree systems to specify the m41t80 as a
wakeup-source.

Signed-off-by: Curtis Klein <curtis.klein@hpe.com>
Link: https://lore.kernel.org/r/20240222011129.79241-1-curtis.klein@hpe.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>