powerpc/85xx: Fix SMP compile error and allow NULL for smp_ops

The following commit introduced a compile error since it removed
the implementation of smp_85xx_basic_setup:

commit 77c0a700
Author: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date:   Fri Aug 28 14:25:04 2009 +1000

    powerpc: Properly start decrementer on BookE secondary CPUs

Make it so that smp_ops probe() and setup_cpu() can be set to NULL.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>