usb: xhci-mtk: add generic compatible string

The xhci-mtk driver is a generic driver for MediaTek xHCI IP, add
a generic compatible to avoid confusion when support new SoCs but
use a compatible with specific SoC's name "mt8173".

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