ARC: fix build warning in devtree

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
diff --git a/arch/arc/kernel/devtree.c b/arch/arc/kernel/devtree.c
index 0b3ef40..fffdb5e 100644
--- a/arch/arc/kernel/devtree.c
+++ b/arch/arc/kernel/devtree.c
@@ -41,7 +41,7 @@
 {
 	const struct machine_desc *mdesc;
 	unsigned long dt_root;
-	void *clk;
+	const void *clk;
 	int len;
 
 	if (!early_init_dt_scan(dt))