dt-bindings: Add a binding for Mediatek xHCI host controller

add a DT binding documentation of xHCI host controller for the
MT8173 SoC from Mediatek.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>