dt-bindings: mt8173-mtu3: add generic compatible and rename file

The mt8173-mtu3.txt actually holds the bindings for all mediatek
SoCs with usb3 DRD IP, so add a generic compatible and change the
name to mediatek,mtu3.txt.

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>