ppc/pnv: add a PnvCore object

This is largy inspired by sPAPRCPUCore with some simplification, no
hotplug for instance. A set of PnvCore objects is added to the PnvChip
and the device tree is populated looping on these cores.

Real HW cpu ids are now generated depending on the chip cpu model, the
chip id and a core mask. The id is propagated to the CPU object, using
properties, to set the SPR_PIR (Processor Identification Register)

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