[MIPS] Hack for SB1 cache issues

Removing flush_icache_page a while ago broke SB1 which was using an empty
flush_data_cache_page function.  This glues things well enough so a more
efficient but also more intrusive solution can be found later.

Signed-Off-By: Thiemo Seufer <ths@networkno.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>