ARC: fix build warning in devtree

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/devtree.c

@@ -41,7 +41,7 @@ const struct machine_desc * __init setup_machine_fdt(void *dt)
 {
 	const struct machine_desc *mdesc;
 	unsigned long dt_root;
-	void *clk;
+	const void *clk;
 	int len;
 
 	if (!early_init_dt_scan(dt))