[MIPS] TX49XX has prefetch.

    
The TX49XX has the prefetch instruction.  It supports only Pref_Load
(hint 0).  Actually changes in this patch except for Kconfig are not
have any effects, I added these changes to prevent misuse of unsupported
hints.
    
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>