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

Make the ref clock optional, then we no need refer to a fixed-clock
in DTS anymore when the clock of USB3 PHY comes from oscillator
directly

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>