ARM: dts: Add rtc_src clk for s3c-rtc on exynos5250-snow

commit 546b117fdf17 ("rtc: s3c: add support for RTC of Exynos3250 SoC")
added an "rtc_src" DT property for the Samsung's S3C Real Time Clock
controller that specifies the 32.768 kHz clock that uses the RTC as
its source clock. In the case of the Exynos5250 based Snow board, the
Maxim 77686 32kHz AP clock is used as the source clock.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>