rtc: s5m: use devm_i2c_new_dummy_device()

Use the managed variant of i2c_new_dummy_device() to shrink code and
remove the goto label. We can drop the remove callback now too.

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/20210114102219.23682-3-brgl@bgdev.pl