ppc/xics: introduce an 'intc' backlink under PowerPCCPU

Today, the ICPState array of the sPAPR machine is indexed with
'cpu_index' of the CPUState. This numbering of CPUs is internal to
QEMU and the guest only knows about what is exposed in the device
tree, that is the 'cpu_dt_id'. This is why sPAPR uses the helper
xics_get_cpu_index_by_dt_id() to do the mapping in a couple of places.

To provide a more generic XICS layer, we need to abstract the IRQ
'server' number and remove any assumption made on its nature. It
should not be used as a 'cpu_index' for lookups like xics_cpu_setup()
and xics_cpu_destroy() do.

To reach that goal, we choose to introduce a generic 'intc' backlink
under PowerPCCPU, and let the machine core init routine do the
ICPState lookup. The resulting object is passed on to xics_cpu_setup()
which does the store under PowerPCCPU. The IRQ 'server' number in XICS
is now generic. sPAPR uses 'cpu_dt_id' and PowerNV will use 'PIR'
number.

This also has the benefit of simplifying the sPAPR hcall routines
which do not need to do any ICPState lookups anymore.

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