rtc: isl12022: implement support for the #clock-cells DT property

rtc: isl12022: implement support for the #clock-cells DT property

If device tree implies that the chip's IRQ/F_OUT pin is used as a
clock, expose that in the driver. For now, pretend it is a
fixed-rate (32kHz) clock; if other use cases appear the driver can be
updated to provide its own clk_ops etc.

When the clock output is not used on a given board, one can prolong
the battery life by ensuring that the FOx bits are 0. For the hardware
I'm currently working on, the RTC draws 1.2uA with the FOx bits at
their default 0001 value, dropping to 0.88uA when those bits are
cleared.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230615105826.411953-9-linux@rasmusvillemoes.dk
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>