ARM: dts: add 'rtc_src' clock to rtc node for exynos4412-odroid boards

The Exynos4412 SoC has a s3c6410 RTC where the source clock
is now a mandatory property.
This patch fixes probe failure of s3c-rtc on Odroid-X2/U2/U3 boards.

Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Tested-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>