rtc: s5m: Cleanup by removing useless 'rtc' prefix from fields

Remove the 'rtc' prefix from some of the fields in struct
s5m_rtc_reg_config because it is obvious - this is a RTC driver. No
functional changes.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>