[POWERPC] mpic: set IPIs to be per-CPU

This patch changes the MPIC IPIs to be per-CPU to avoid getting a
warning ("Cannot set affinity for irq 251") when taking a CPU
offline via sysfs or during suspend.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>