rtc: s5m: prepare for external regmap

rtc: s5m: prepare for external regmap

The Samsung S2MPG10 PMIC is not connected via I2C as this driver
assumes, hence this driver's current approach of creating an I2C-based
regmap doesn't work for it, and this driver should use the regmap
provided by the parent (core) driver instead for that PMIC.

To prepare this driver for s2mpg support, restructure the code to only
create a regmap if one isn't provided by the parent.

No functional changes, since the parent doesn't provide a regmap for
any of the PMICs currently supported by this driver. Having this change
separate will simply make the addition of S2MPG10 support more
self-contained, without additional restructuring.

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-v4-26-d66d5f39b6bf@linaro.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>