ppc/xics: Move SPAPR specific code to a separate file

Leave the core ICP/ICS logic in xics.c and move the top level
class wrapper, hypercall and RTAS handlers to xics_spapr.c

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[add cpu.h in xics_spapr.c, move set_nr_irqs and set_nr_servers to
 xics_spapr.c]
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>