rtc: tps65910: Support wakeup-source property

TPS65910 is a PMIC MFD device and RTC is one of its functions. The
wakeup-source DT property is specified for the parent MFD device and we
need to use this property for the RTC in order to allow to use RTC alarm
for waking up system from suspend by default, instead of requiring user
to enable wakeup manually via sysfs.

Tested-by: Peter Geis <pgwipeout@gmail.com>
Tested-by: Matt Merhar <mattmerhar@protonmail.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210120211603.18555-1-digetx@gmail.com