ppc/pnv: add a core mask to PnvChip

This will be used to build real HW ids for the cores and enforce some
limits on the available cores per chip.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
