[POWERPC] spufs: runqueue simplification

This is the biggest patch in this series, and it reworks the guts of
the spu scheduler runqueue mechanism:

 - instead of embedding a waitqueue in the runqueue there is now a
   simple doubly-linked list, the actual wakeups happen by reusing
   the stop_wq in the spu context (maybe we should rename it one day)
 - spu_free and spu_prio_wakeup are merged into a single spu_reschedule
   function
 - various functionality is split out into small helpers, and kerneldoc
   comments are added in various places to document what's going on.
 - spu_activate is rewritten into a tight loop by removing test for
   various impossible conditions and using the infrastructure in this
   patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>