ARM: dts: Enable RTC node for Arndale

The issues reported in commit 522ccdb6 ("ARM: dts: Disable the RTC
by default on exynos5") are no longer reproduced on EXYNOS5250 based
Arndale board. Hence re-enabling RTC support for Arndale board.

This is helpful for testing S2R on Arndale board.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>