rtc: sirfsoc: move to regmap APIs from platform-specific APIs

The current codes use CSR platform specific API exported by machine
codes to read/write RTC registers. they are:
sirfsoc_rtc_iobrg_readl()
sirfsoc_rtc_iobrg_writel()

commit b1999477 ("ARM: prima2: move to use REGMAP APIs for rtciobrg")
moves to regmap support, now we can move to use regmap APIs in RTC
driver.

Signed-off-by: Guo Zeng <guo.zeng@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>