dt-bindings: usb: usb-drd: Describe default dual-role mode

The dual-role mode default, in the absence of the dr_mode property, is
already documented to be OTG. Use the "default" property to mark it as
such more explicitly.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221103115923.1467525-1-thierry.reding@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb-drd.yaml

@@ -27,6 +27,7 @@ properties:
       should default to OTG.
     $ref: /schemas/types.yaml#/definitions/string
     enum: [host, peripheral, otg]
+    default: otg
 
   hnp-disable:
     description: