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

Sometimes the reference clock of USB3 PHY comes from oscillator
directly, and no need refer to a fixed-clock in DTS anymore
if make it optional.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>