clk: mediatek: Add USB clock support in MT8173 APMIXEDSYS

Add REF2USB_TX clock support into MT8173 APMIXEDSYS. This clock
is needed by USB 3.0.

Signed-off-by: James Liao <jamesjj.liao@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
diff --git a/drivers/clk/mediatek/clk-mtk.h b/drivers/clk/mediatek/clk-mtk.h
index 24b73ff..32d2e45 100644
--- a/drivers/clk/mediatek/clk-mtk.h
+++ b/drivers/clk/mediatek/clk-mtk.h
@@ -180,6 +180,9 @@
 		const struct mtk_pll_data *plls, int num_plls,
 		struct clk_onecell_data *clk_data);
 
+struct clk *mtk_clk_register_ref2usb_tx(const char *name,
+			const char *parent_name, void __iomem *reg);
+
 #ifdef CONFIG_RESET_CONTROLLER
 void mtk_register_reset_controller(struct device_node *np,
 			unsigned int num_regs, int regofs);