[MIPS] cpu/mips/cache.S: Add dcache_enable

Recent bootelf command fixes (017e9b79,
"allow ports to override bootelf behavior") requires ports to have this
function.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>