dt-bindings: Add usb3.0 phy binding for MT65xx SoCs

add a DT binding documentation of usb3.0 phy for MT65xx
SoCs from Mediatek.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>