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: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>