dt-bindings: usb: mediatek,mtu3: Use graph schema

Users of the DT graph binding now should reference it for their 'port'
schemas, so add it for Mediatek MTU3 binding.

Cc: Chunfeng Yun <chunfeng.yun@mediatek.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210327191448.410795-1-robh@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>