Blackfin: fix unused clk var warning

This is only used on BF60x code (so this patch should get squashed into
the original one that added it).
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>

arch/blackfin/kernel/setup.c

@@ -892,7 +892,9 @@ void __init setup_arch(char **cmdline_p)
 {
 	u32 mmr;
 	unsigned long sclk, cclk;
+#ifdef CONFIG_BF60x
 	struct clk *clk;
+#endif
 
 	native_machine_early_platform_add_devices();