ARM: dts: Explicitly set dr_mode on exynos5250-snow

Explicitly set the dr_mode for the dwc3 controller on the
Snow board to host mode. This is required to ensure the
controller is initialized in the right mode if the kernel is
build with USB gadget support.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>