rtc: pcf2127: adapt for time/date registers at any offset

This will simplify the implementation of new variants into this driver.

Some variants (PCF2131) have a 100th seconds register. This register is
currently not supported in this driver.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20230622145800.2442116-7-hugo@hugovil.com


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