powerpc/cpumask: Convert mpic driver to new cpumask API

Convert to the new cpumask API.

irq_choose_cpu can be simplified by using cpumask_next and cpumask_first.

smp_mpic_message_pass was doing open coded cpumask manipulation and passing an
int for a cpumask into mpic_send_ipi. Since mpic_send_ipi is only used
locally, make it static and convert it to take a cpumask. This allows us
to clean up the mess in smp_mpic_message_pass.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>