ARM: dts: Enable RTC by default on EXYNOS5440

RTC is a basic feature of EXYNOS5440. But it was disabled
by issue that comes from EXYNOS5250. (commit id: 522ccdb6

)
That issue only applies to EXYNOS5250 not EXYNOS5440. So,
there is no issue with enabled RTC on EXYNOS5440. This
change was tested on EXYNOS5440.

Signed-off-by: Youngmin Nam <youngmin.nam@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>