dt-bindings: usb-connector: Use OF graph schema

Now that we have a graph schema, reference it from the usb-connector
schema.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20201102203656.220187-3-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>