This exports some clock functions used by some modules.
This fixes this linking problem:

ERROR: "clk_set_rate" [drivers/usb/phy/phy-generic.ko] undefined!
ERROR: "clk_round_rate" [drivers/media/v4l2-core/videodev.ko] undefined!
ERROR: "clk_set_rate" [drivers/media/v4l2-core/videodev.ko] undefined!

In the upstream kernel it is fixed here:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=411520af8ec9456886359b42628e583ac58e7e44

--- a/arch/mips/ath79/clock.c
+++ b/arch/mips/ath79/clock.c
@@ -488,3 +488,15 @@ unsigned long clk_get_rate(struct clk *c
 	return clk->rate;
 }
 EXPORT_SYMBOL(clk_get_rate);
+
+int clk_set_rate(struct clk *clk, unsigned long rate)
+{
+	return 0;
+}
+EXPORT_SYMBOL_GPL(clk_set_rate);
+
+long clk_round_rate(struct clk *clk, unsigned long rate)
+{
+	return 0;
+}
+EXPORT_SYMBOL_GPL(clk_round_rate);