rtc: s5m.c: Add support for S2MPS15 RTC

RTC found in s2mps15 is almost same as one found on s2mps13
with few differences in RTC_UPDATE register fields, like:
1> Bit[4] and Bit[1] are reversed
   - On s2mps13
          WUDR -> bit[4], AUDR -> bit[1]
   - On s2mps15
	  WUDR -> bit[1], AUDR -> bit[4]
2> In case of s2mps13, for alarm register, need to set both
   WDUR and ADUR high, whereas for s2mps15 only set AUDR to high.
3> On s2mps15, WUDR, RUDR and AUDR functions should never be used
   at the same time.

This patch add required changes to enable s2mps15 rtc timer.

Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>