powerpc/mpic: Add support for discontiguous cores

There is one place in the MPIC driver that assumes that the cores are numbered
from 0 to n-1.  However, this is not true if the CPUs are not numbered
sequentially.  This can happen on a eight-core SOC where cores two and three
are removed in the device tree.  So instead of blindly looping, we iterate
over the discovered CPUs and use the SMP ID as the index.

This means that we no longer ask the MPIC how many CPUs there are, so
we also delete mpic->num_cpus.

We also catch if the number of CPUs in the SOC exceeds the number that the
MPIC supports.  This should never happen, of course, but it's good to be
sure.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>