rtc: s3c: Remove usage of devm_rtc_device_register()

devm_rtc_device_register() is deprecated. Use devm_rtc_allocate_device()
and devm_rtc_register_device() API instead. This change doesn't change
the behavior, but allows for further improvements.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211021202256.28517-2-semen.protsenko@linaro.org