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 <[e-mail]>
Signed-off-by: Kishon Vijay Abraham I <[e-mail]>

drivers/phy/mediatek/phy-mtk-tphy.c

···
 		if (tphy->u3phya_ref)
 			continue;
 
-		instance->ref_clk = devm_clk_get(&phy->dev, "ref");
+		instance->ref_clk = devm_clk_get_optional(&phy->dev, "ref");
 		if (IS_ERR(instance->ref_clk)) {
 			dev_err(dev, "failed to get ref_clk(id-%d)\n", port);
 			retval = PTR_ERR(instance->ref_clk);