ARC: fix linux-next build breakage

| ~/linux/arch/arc/kernel/setup.c: In function show_cpuinfo:
| ~/linux/arch/arc/kernel/setup.c:463:9: error: implicit declaration of function of_find_node_by_name [-Werror=implicit-function-declaration]
|  struct device_node *core_clk = of_find_node_by_name(NULL, "core_clk");

Reported-by: Anton Kolesov <akolesov@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/setup.c

@@ -14,6 +14,7 @@
 #include <linux/module.h>
 #include <linux/cpu.h>
 #include <linux/of_fdt.h>
+#include <linux/of.h>
 #include <linux/of_platform.h>
 #include <linux/cache.h>
 #include <asm/sections.h>