MIPS: Loongson-3: support irq_set_affinity() in i8259 chip

MIPS: Loongson-3: support irq_set_affinity() in i8259 chip

With this patch we can set irq affinity via procfs, so as to improve
network performance.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J . Hill <Steven.Hill@cavium.com>
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16590/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>