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>