ARM: dts: Use define for s3c-rtc clock id for exynos3250-monk

Use a define instead of raw number as a ID for "rtc_src" clock.

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>