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: NChunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NRyder Lee <ryder.lee@mediatek.com>

drivers/phy/phy-mtk-tphy.c

@@ -338,7 +338,8 @@ static int mtk_tphy_probe(struct udevice *dev)
 		tphy->phys[index] = instance;
 		index++;
 
-		err = clk_get_by_index_nodev(subnode, 0, &instance->ref_clk);
+		err = clk_get_optional_nodev(subnode, "ref",
+					     &instance->ref_clk);
 		if (err)
 			return err;
 	}