pseries: Move construction of /interrupt-controller fdt node

pseries: Move construction of /interrupt-controller fdt node

Currently the device tree node for the XICS interrupt controller is in
spapr_create_fdt_skel().  As part of consolidating device tree construction
to reset time, this moves it to a function called from spapr_build_fdt().

In addition we move the actual code into hw/intc/xics_spapr.c with the
rest of the PAPR specific interrupt controller code.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>