powerpc/mpic: Simplify ipi cpu mask handling

Now that MSG_ALL and MSG_ALL_BUT_SELF have been eliminated,
smp_mpic_mesage_pass no longer needs to lookup the cpumask just to
have mpic_send_ipi extract part of it and recode it in a NR_CPUS loop
by mpic_physmask.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>