ARM: dts: Enable S3C RTC on exynos4412-trats2 and exynos5420-arndale-octa

Extend the S3C RTC node with rtc_src clock so it could be operational.
The rtc_src clock is provided by MAX77686 (Trats2) or S2MPS11 (Arndale
Octa).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>