usb: xhci-mtk: add reference clock

usually, the reference clock comes from 26M oscillator directly,
but some SoCs are not, add it for compatibility.

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