MIPS: BCM63XX: drop SYS_HAS_CPU_MIPS32R1

All MIPS cores on BCM63XX identify as Broadcom, not MIPS, so no need
to support non-broadcom MIPS CPUs. This also ensures that CPU_BMIPS
is always selected.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6246/