powerpc: Move smp_ops_t from machdep.h to smp.h

I can't see any reason these functions are needed by machdep.h
and they are all hidden by CONFIG_SMP with no UP alternative.

Also move the declarations for the fallback timebase ops, which
are used to fill in the smp ops.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>