rtc: s5m: add support for S2MPG10 RTC

rtc: s5m: add support for S2MPG10 RTC

Add support for Samsung's S2MPG10 PMIC RTC, which is similar to the
existing PMIC RTCs supported by this driver.

S2MPG10 doesn't use I2C, so we expect the core driver to have created a
regmap for us.

Additionally, it can be used for doing a cold-reset. If requested to do
so (via DT), S2MPG10 is programmed with a watchdog configuration that
will perform a full power cycle upon watchdog expiry.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250409-s2mpg10-v4-27-d66d5f39b6bf@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>