rtc: s3c: Extract read/write IO into separate functions

Create dedicated functions for I/O operations and BCD conversion. It can
be useful to separate those from representation conversion and other
stuff found in RTC callbacks.

This patch does not introduce any functional changes, it's merely
refactoring change.

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/20211021202256.28517-3-semen.protsenko@linaro.org