powerpc/ps3: Fix SMP lockdep boot warning

Move the PS3 IPI message setup from ps3_smp_setup_cpu() to ps3_smp_probe().

Fixes startup warnings like these:

  ------------[ cut here ]------------
  WARNING: at kernel/lockdep.c:2649
  Modules linked in:
  ...
  ---[ end trace 31fd0ba7d8756001 ]---

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>