Factor out common BMIPS options into "CPU_BMIPS".  Add L2 cache for
BMIPS5000.  Add CPU_MIPS32 to satisfy checks in page.h, r4k_switch.S,
tlb-r4k.c, etc.
Signed-off-by: NKevin Cernekee <cernekee@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2953/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>