arm64: dts: Fix RTC by providing rtc_src clock

Add RTC source clock as Exynos7 needs source (32.768KHz) clock
for RTC block. Without this currently S3C RTC driver probe is broken
on this SoC.

Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>