rtc: isl12022: Prepare for extending rtc device drvdata

Add a struct to hold the regmap pointer, so more information can be added.

This is morally a revert of commit f525b210 ("rtc: isl12022: Get rid of
unneeded private struct isl12022").

Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Esben Haabendal <esben@geanix.com>
Link: https://lore.kernel.org/r/20240913-rtc-isl12022-alarm-irq-v2-1-37309d939723@geanix.com


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