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.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>