[POWERPC] Cleanup pseries smp initialisation code

Move some extern declarations from setup.c into the new pseries.h.
While we're at it, provide dummy implementations for !SMP, to avoid
cluttering the C file with more #ifdefs.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>