MIPS: Install handlers for software IRQs

BMIPS4350/4380/5000 CMT/SMT all use SW INT0/INT1 for inter-thread
signaling.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1709/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>