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>