MIPS: BMIPS: select CPU_HAS_PREFETCH

As they are MIPS32 CPUs they do support the prefetch opcode.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6243/