rtc: s5m: replace open-coded read/modify/write registers with regmap helpers

rtc: s5m: replace open-coded read/modify/write registers with regmap helpers

Instead of the open-coded read/modify/write sequence, we can simply use
the regmap helpers regmap_set_bits() and regmap_update_bits()
respectively.

This makes the code easier to read, and avoids extra work in case the
underlying bus supports updating bits via
struct regmap_bus::reg_update_bits() directly (which is the case for
S2MPG10 on gs101 where this driver communicates via ACPM).

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250409-s2mpg10-v4-31-d66d5f39b6bf@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>