rtc: isl12022: implement RTC_VL_READ ioctl

Hook up support for reading the values of the SR_LBAT85 and SR_LBAT75
bits. Translate the former to "battery low", and the latter to
"battery empty or not-present".

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230615105826.411953-6-linux@rasmusvillemoes.dk


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