[POWERPC] Don't enable cpu hotplug on pSeries machines with MPIC

Don't allow cpu hotplug on systems lacking XICS interrupt controller
(i.e. with an MPIC interrupt controller), since the current pSeries
platform code is hardcoded for XICS.

This works around the bug reported by Paul Mackerras where the
disable_nonboot_cpus() call recently added to the shutdown path will
cause an oops on older pSeries machines.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>