MIPS: BMIPS: Add CFLAGS, Makefile entries for BMIPS

Add CONFIG_CPU_BMIPS* in all of the right places, so that BMIPS kernel
images will compile and run.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2955/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>