ppc/xics: introduce an icp_create() helper

ppc/xics: introduce an icp_create() helper

The sPAPR and the PowerNV core objects create the interrupt presenter
object of the CPUs in a very similar way. Let's provide a common
routine in which we use the presenter 'type' as a child identifier.

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