Documentation: dt-bindings: add dt binding info for dwc2 dr_mode

Indicate that the generic dr_mode binding should be used for dwc2.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/dwc2.txt

@@ -18,6 +18,8 @@ Optional properties:
 - phys: phy provider specifier
 - phy-names: shall be "usb2-phy"
 Refer to phy/phy-bindings.txt for generic phy consumer properties
+- dr_mode: shall be one of "host", "peripheral" and "otg"
+  Refer to usb/generic.txt
 
 Example: