rtc: 88pm860x: stop setting a default time

It doesn't make sense to set the RTC to a default value at probe time. Let
the core handle invalid date and time.

Link: https://lore.kernel.org/r/20200311223956.51352-2-alexandre.belloni@bootlin.com


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