ARM: dts: Re-organize RTC status for exynos5250

Some boards might not have RTC xtal, so RTC shouldn't really be
enabled by default. Enable it in the required board files. For
now enable it for all board files to maintain the enabled status
exisitng prior to this patch.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>