ppc/pnv: create the ICP object under PnvCore

ppc/pnv: create the ICP object under PnvCore

Each thread of a core is linked to an ICP. This allocates a PnvICPState
object before the PowerPCCPU object is realized and lets the XICSFabric
do the store under the 'intc' backlink when xics_cpu_setup() is
called.

This modeling removes the need of maintaining an array of ICP objects
under the PowerNV machine and also simplifies the XICSFabric icp_get()
handler.

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>