MIPS: Lemote 2F: Add IRQ support

The generic i8259_irq() will make kernel hang on booting, so Loongson 2F
needs its own polling method.

IP6 is shared by the bonito interrupt and perfcounter interrupts.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: zhangfx@lemote.com
Cc: yanh@lemote.com 
Cc: huhb@lemote.com
Cc: Nicholas Mc Guire <hofrat@hofr.at>
Cc: Arnaud Patard <apatard@mandriva.com>
Cc: loongson-dev@googlegroups.com
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>