rtc: s5m: select REGMAP_I2C

The rtc-s5m uses the I2C regmap but doesn't select it in Kconfig so
depending on the configuration the build may fail. Fix it.

Fixes: 959df777 ("rtc: Enable compile testing for Maxim and Samsung drivers")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210114102219.23682-2-brgl@bgdev.pl