phy: phy-mtk-tphy: make ref clock optional

phy: phy-mtk-tphy: make ref clock optional

If make the ref clock optional, no need refer to fixed-clock when
the ref clock is always on or comes from oscillator directly.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>