Make Octeon GPIO IRQ chip CPU hotplug-aware

Seems that irq_cpu_offline callbacks were forgotten in v1 and v2 CIU
GPIO chips. There is such a callback for octeon_irq_chip_ciu2_gpio,
covering CIU2 chips. Without this callback GPIO IRQs are not being migrated
during core offlining. Patch is tested on Octeon II.
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nsn.com>
Cc: David Daney <ddaney.cavm@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8201/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>