dt-bindings: usb: snps,dwc3: document extcon property

The DWC3 USB Linux driver and several DTS use extcon property, so
document it, even though it is deprecated, to fix warnings like:

  sm6125-sony-xperia-seine-pdx201.dtb: usb@4ef8800: usb@4e00000: Unevaluated properties are not allowed ('extcon' was unexpected)
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230304130302.51497-1-krzysztof.kozlowski@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/snps,dwc3.yaml

@@ -70,6 +70,10 @@ properties:
 
   dma-coherent: true
 
+  extcon:
+    maxItems: 1
+    deprecated: true
+
   iommus:
     maxItems: 1