powerpc/mm: Rework context management for CPUs with no hash table

This reworks the context management code used by 4xx,8xx and
freescale BookE.  It adds support for SMP by implementing a
concept of stale context map to lazily flush the TLB on
processors where a context may have been invalidated.  This
also contains the ground work for generalizing such lazy TLB
flushing by just picking up a new PID and marking the old one
stale.  This will be implemented later.

This is a first implementation that uses a global spinlock.

Ideally, we should try to get at least the fast path (context ID
already assigned) lockless or limited to a per context lock,
but for now this will do.

I tried to keep the UP case reasonably simple to avoid adding
too much overhead to 8xx which does a lot of context stealing
since it effectively has only 16 PIDs available.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>