rtc: s3c: add support for RTC of Exynos3250 SoC

Add support for RTC of Exynos3250 SoC.  The Exynos3250 needs source
clock(32.768KHz) for RTC block.  If source clock of RTC is registerd on
clock list of common clk framework, Exynos RTC drvier have to control
this clock.

Clock list for s3c-rtc device:
- rtc : CLK_RTC of CLK_GATE_IP_PERIR is gate clock for RTC.
- rtc_src : XrtcXTI is 32.768.kHz source clock for RTC.
 (XRTCXTI: Specifies a clock from 32.768 kHz crystal pad with XRTCXTI and
 XRTCXTO pins. RTC uses this clock as the source of a real-time clock.)

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>