ARM: dts: add usb 2.0 clock references to exynos5250 device tree

This is a fixup to two device tree nodes that have already landed but
without clock nodes since the transition to common clock happened at
the same time.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
[gautam.vivek@samsung.com: tested on smdk5250]
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>