ARM: dts: Enable RTC node for exynos5250-snow

By default the exynos RTC is disabled.  Enable it for snow.  There's
also an external RTC on the max77686 PMIC but we haven't yet enabled
that.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>