[POWERPC] Fix non-MPIC CHRPs with CONFIG_SMP set

Pseudo-CHRP machines like Pegasos without an MPIC would crash at boot if
CONFIG_SMP was set because the "smp_ops" pointer was set to MPIC related
ops unconditionally. This patch makes it NULL on machines that don't
support SMP and provides proper default behaviour in the callers when
smp_ops is NULL.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>