rtc: add driver for Marvell 88PM886 PMIC RTC

rtc: add driver for Marvell 88PM886 PMIC RTC

RTC lives on the chip's base register page. Add the relevant register
definitions and implement a basic set/read time functionality. Tested
with the samsung,coreprimevelte smartphone which contains this PMIC and
whose vendor kernel tree has also served as the sole reference for this.

Signed-off-by: Karel Balej <balejk@matfyz.cz>
Acked-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20241012193345.18594-2-balejk@matfyz.cz
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>