[PATCH v2] dt-bindings: usb: dwc2: document the port when usb-role-switch is used

[Fabrice Gasnier <fabrice.gasnier@foss.st.com>]

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.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
---
Changes in v2:
Rob's review comments:
- drop reference to graph.txt
- drop unneeded quotes
---
 Documentation/devicetree/bindings/usb/dwc2.yaml | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/Documentation/devicetree/bindings/usb/dwc2.yaml b/Documentation/devicetree/bindings/usb/dwc2.yaml
index 10c7d9b..a351f3c 100644
--- a/Documentation/devicetree/bindings/usb/dwc2.yaml
+++ b/Documentation/devicetree/bindings/usb/dwc2.yaml
@@ -130,6 +130,16 @@ 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
-- 
2.7.4