rtc: isl12022: add support for trip level DT binding

Implement support for using the values given in the
isil,battery-trip-levels-microvolt property to set appropriate values
in the VB85TP/VB75TP bits in the PWR_VBAT register.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230615105826.411953-5-linux@rasmusvillemoes.dk


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