ARM: dts: Re-organize RTC status for exynos5420

Some boards might not have RTC xtal, so RTC shouldn't really be
enabled by default. Enable it in the required board files.

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