powerpc/mpic: Use new smp_request_message_ipi

MPIC has 4 ipis, so it can use the new smp_request_message_ipi to
reduce pathlength when receiving an ipi.

This has the side effect of using the common ipi names, and also
continuing to try request the remaining messages when one fails.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>