ARM: dts: Update the "status" property of RTC DT node for Exynos5250 SoC

Moves the RTC DT node's "status" property from exynos5250 board
(arndale & snow) dts files to exynos5250.dtsi, since the bindings
in exynos5250.dtsi depicts the RTC h/w completely.

Signed-off-by: Vikas Sajjan <vikas.sajjan@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>