arm: dts: imx6qdl-gw*: add dr_mode prop to dt to avoid error

The fsl-usb dt bindings in Linux default dr_mode to 'host' for
backward compatibility however U-Boot prints an error if
this property does not exist. Declare it in the Gateworks
Ventana device-trees to avoid the error.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>