MIPS: BMIPS: merge CPU options into one option

Instead of treating each flavour as an exclusive CPU to select, make
BMIPS the only option and let SYS_HAS_CPU_BMIPS* decide for which
flavours to include support.

Run tested on BMIPS3300 and BMIPS4350, only build tested for BMIPS4380
and BMIPS5000.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6240/