spapr: populate device tree depending on XIVE_EXPLOIT option

When XIVE is supported, the device tree should be populated
accordingly and the XIVE memory regions mapped to activate MMIOs.

Depending on the design we choose, we could also allocate different
ICS and ICP objects, or switch between objects. This needs to be
discussed.

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