irqchip/loongson: Select GENERIC_IRQ_EFFECTIVE_AFF_MASK if SMP for IRQ_LOONGARCH_CPU

An interrupt's effective affinity can only be different from its configured
affinity if there are multiple CPUs. Make it clear that this option is only
meaningful when SMP is enabled. Otherwise, there exists "WARNING: unmet
direct dependencies detected for GENERIC_IRQ_EFFECTIVE_AFF_MASK" when make
menuconfig if CONFIG_SMP is not set on LoongArch.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240326121130.16622-3-yangtiezhu@loongson.cn