dt-bindings: mt8173-mtu3: add devicetree bindings

add a DT binding doc for MediaTek USB3 DRD driver

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>