dt-bindings: usb: dwc2: document the port when usb-role-switch is used

Document the "port" property, which is used with "usb-role-switch"
to describe the bus connector.
Definition is inspired from mediatek,mtu3.yaml.

This fixes some errors seen when running "make dtbs_check":
... 'port' does not match any of the regexes: 'pinctrl-[0-9]+'
        From schema: ... Documentation/devicetree/bindings/usb/dwc2.yaml

Note: add dependencies so that 'usb-role-switch' without port is valid or
both must be present.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/1635336752-14796-1-git-send-email-fabrice.gasnier@foss.st.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/dwc2.yaml

@@ -136,6 +136,16 @@ properties:
     description: If present indicates that we need to reset the PHY when we 
       detect a wakeup. This is due to a hardware errata.
 
+  port:
+    description:
+      Any connector to the data bus of this controller should be modelled
+      using the OF graph bindings specified, if the "usb-role-switch"
+      property is used.
+    $ref: /schemas/graph.yaml#/properties/port
+
+dependencies:
+  port: [ usb-role-switch ]
+
 required:
   - compatible
   - reg