dt-bindings: usb: snps,dwc3: document HS & SS OF graph ports

Document the optional ports subnode to describe the High-Speed
and Super-Speed connections as separate OF graph links.

The ports property is an alternative to the already documented
single port subnode property.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230130-topic-sm8450-upstream-pmic-glink-v5-5-552f3b721f9e@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>