spapr: Abstract CPU core device and type specific core devices

Add sPAPR specific abastract CPU core device that is based on generic
CPU core device. Use this as base type to create sPAPR CPU specific core
devices.

TODO:
- Add core types for other remaining CPU types
- Handle CPU model alias correctly

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>