irqchip/loongson-eiointc: Set CPU affinity only on SMP machines for LoongArch

According to the code comment of "struct irq_chip", the member
"irq_set_affinity" is to set the CPU affinity on SMP machines, so define
and call eiointc_set_irq_affinity() only under CONFIG_SMP.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240326121130.16622-4-yangtiezhu@loongson.cn