powerpc: Use nr_cpu_ids in initial paca allocation

Now that we never set a cpu above nr_cpu_ids possible we can
limit our initial paca allocation to nr_cpu_ids.  We can then
clamp the number of cpus in platforms/iseries/setup.c.

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