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

The mt8173-xhci.txt actually holds the bindings for all mediatek
SoCs with xHCI controller, so add a generic compatible and change
the name to mediatek,mtk-xhci.txt to reflect that.

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