powerpc/mpic: allow coreint to be determined by MPIC version

This will be used by the qemu-e500 platform, as the MPIC version (and
thus whether we have coreint) depends on how QEMU is configured.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>