ARM: dts: add 'rtc_src' clock to rtc node for exynos5422-odroidxu3

The Exynos5422 SoC has a s3c6410 RTC where the source clock
is now a mandatory property.

This patch fixes probe failure of s3c-rtc on Odroid-XU3 boards.

Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Tested-by: Anand Moon <linux.amoon@gmail.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>