xics_system_init() does not need 'nr_servers' anymore as it is only
used to define the 'interrupt-controller' node in the device tree. So
let's just compute the value when calling spapr_dt_xics().

This also gives us an opportunity to simplify the xics_system_init()
routine and introduce a specific spapr_ics_create() helper to create
the sPAPR ICS object.

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