csky/smp: Fixup boot failed when CONFIG_SMP

If we use a non-ipi-support interrupt controller, it will cause panic here.
We should let cpu up and work with CONFIG_SMP, when we use a non-ipi intc.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

arch/csky/kernel/smp.c

@@ -120,7 +120,7 @@ void __init setup_smp_ipi(void)
 	int rc;
 
 	if (ipi_irq == 0)
-		panic("%s IRQ mapping failed\n", __func__);
+		return;
 
 	rc = request_percpu_irq(ipi_irq, handle_ipi, "IPI Interrupt",
 				&ipi_dummy_dev);