[MIPS] Fix deadlock on MP with cache aliases.

A proper fix would involve introducing the notion of shared caches but
at this stage of 2.6.17 that's going to be too intrusive and not needed
for current hardware; aside I think some discussion will be needed.

So for now on the affected SMP configurations which happen to suffer from
cache aliases we make use of the fact that a single cache will be shared
by all processors.  This solves the deadlock issue and will improve
performance by getting rid of the smp_call_function overhead.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>