dt-bindings: usb: Use OF graph schema

Now that we have a graph schema, rework the USB related schemas to use
it. Mostly this is adding a reference to graph.yaml and dropping duplicate
parts from schemas.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: linux-usb@vger.kernel.org
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20210112153527.391232-1-robh@kernel.org


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