[MIPS] Allow setting of the cache attribute at run time.

Slightly tacky, but there is a precedent in the sparc archirecture code.

Signed-off-by: Chris Dearman <chris@mips.com>
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>