ARM: dts: exynos: Add missing clocks to RTC node for Arndale board

Add missing clocks to SoC build-in RTC device to make it fully
operational on Exynos5250-based Arndale board.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>