powerpc: Use cpu_thread_in_core in smp_init for of_spin_map

We used to assume that even numbered threads were the primary
threads, ie those that would be listed and started as a cpu from
open firmware.  Replace a left over is even (% 2) check with a check
for it being a primary thread and update the comments.

Tested with a debug print on pseries, identical code found for cell.

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