ARM: dts: Document DP clock in samsung,exynos5-dp binding

The exynos5-dp node needs a clock specified using the common clock
framework.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>