dt-bindings: usb: Add downstream facing ports to realtek binding

Add a graph with 4 output endpoints to this hub binding to support the
scenario where a downstream facing port is connected to a device that
isn't a connector or a USB device with a VID:PID. This will be used to
connect downstream facing ports to USB type-c switches so the USB
superspeed and high speed lanes can be put onto USB connectors.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: Conor Dooley <conor+dt@kernel.org>
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: linux-usb@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: Pin-yen Lin <treapking@chromium.org>
Cc: maciek swiech <drmasquatch@google.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20240223005823.3074029-2-swboyd@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>