[MIPS] i8259: Add disable method.

After 76d21601, the qemu NE2000 was
frequently producing WATCHDOG timeouts.

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>