ARM: dts: Explicitly set dr_mode on exynos5420-arndale-octa

Explicitly set the dr_mode for the second dwc3 controller on the
Arndale Octa 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.

Reported-By: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>