spapr: move the IRQ server number mapping under the machine

This is the second step to abstract the IRQ 'server' number of the
XICS layer. Now that the prereq cleanups have been done in the
previous patch, we can move down the 'cpu_dt_id' to 'cpu_index'
mapping in the sPAPR machine 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>