usb: mtu3: add reference clock

usually, the reference clock comes from 26M oscillator directly,
but some SoCs are not, add it for compatibility.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/usb/mtu3/mtu3.h b/drivers/usb/mtu3/mtu3.h
index ba9df71..aa6fd6a 100644
--- a/drivers/usb/mtu3/mtu3.h
+++ b/drivers/usb/mtu3/mtu3.h
@@ -225,6 +225,7 @@
 	/* common power & clock */
 	struct regulator *vusb33;
 	struct clk *sys_clk;
+	struct clk *ref_clk;
 	/* otg */
 	struct otg_switch_mtk otg_switch;
 	enum usb_dr_mode dr_mode;