MIPS: allow asm/cpu.h to be included from assembly

Add guards around the enum to allow including cpu.h from assembly.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6238/