arm64: dts: mt7622: add usb device nodes

add xhci node and usb3 phy nodes

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Tested-by: Jumin Li <jumin.li@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>