ARM: dts: Document usb clocks in samsung,exynos4210-ehci/ohci bindings

The exynox4210-ehci and exynos4210-ohci nodes need a clock specified
using the common clock framework.  Document it.

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