rtc: pcf2123: let the core handle range offsetting

Set the RTC range properly and use the core windowing and offsetting to
(unfortunately) map back to a 1970-2069 range.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20190819182656.29744-8-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>