powerpc/spufs: better placement of spu affinity reference context

This patch adjusts the placement of a reference context from
a spu affinity chain. The reference context can now be placed
only on nodes that have enough spus not intended to be used by
another gang (already running on the node).

Signed-off-by: Andre Detsch <adetsch@br.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>