MIPS: BCM63XX: let the individual SoCs select the appropriate CPUs

Let each supported chip select the appropirate SYS_HAS_CPU_BMIPS*
option for its embedded processor, so support will be conditionally
included.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6250/