ppc/xics: store the ICS object under the sPAPR machine

ppc/xics: store the ICS object under the sPAPR machine

A list of ICS objects was introduced under the XICS object for the
PowerNV machine but, for the sPAPR machine, it brings extra complexity
as there is only a single ICS. To simplify the code, let's add the ICS
pointer under the sPAPR machine and try to reduce the use of this list
where possible.

Also, change the xics_spapr_*() routines to use an ICS object instead
of an XICSState and change their name to reflect that these are
specific to the sPAPR ICS object.

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