spapr, xics, xive: Move dt_populate from SpaprIrq to SpaprInterruptController

spapr, xics, xive: Move dt_populate from SpaprIrq to SpaprInterruptController

This method depends only on the active irq controller.  Now that we've
formalized the notion of active controller we can dispatch directly
through that, rather than dispatching via SpaprIrq with the dual
version having to do a second conditional dispatch.

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