rtc: pm8xxx: use u32 for timestamps

The PMIC RTC registers are 32-bit so explicitly use u32 rather than
unsigned long for timestamps to reflect the hardware.

This will also help avoid unintentional range extensions when adding
support for managing an external offset.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20230202155448.6715-13-johan+linaro@kernel.org


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