rtc: rv8803: factor out existing register initialization to function

The driver probe currently initializes some registers to non-POR
values. These values are not reinstated if the RTC experiences voltage
loss later on. Prepare for fixing this by factoring out the
initialization to a separate function.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220426071056.1187235-2-s.hauer@pengutronix.de