[MIPS] Use real cache invalidate

R10k non coherent machines need a real dma cache invalidate to get rid of
speculative stores in cache.  For other machines this promises a slight
speedup.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>