MIPS: Octeon: Add irq handlers for GPIO interrupts.

This is needed for follow-on on patches for Octeon to use the Device
Tree to configure GPIO interrupts.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc:  linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3932/


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