s5j/rtc: rework rtc driver

s5j/rtc: rework rtc driver

This commit completely rewrites the RTC driver. Unnecessary code lines
were removed and overcall callflow were designed, following the
conventions at upstream.

As a result, overall binary footprint reduced, when CONFIG_RTC_ALARM=n:

    Before
       text    data     bss     dec filename
       1736       4       1    1741 s5j_rtc.o

    After
       text    data     bss     dec filename
       1248       0       1    1249 s5j_rtc.o
        108       4       0     112 s5j_rtc_lowerhalf.o

Change-Id: Ic641828546827cdff51f8bbf2ee05eb3f501ac88
Signed-off-by: Heesub Shin <heesub.shin@samsung.com>