MIPS: BCM63XX: rework chip detection

Instead of trying to use a correlation of cpu prid and chip id and
hoping they will always be unique, use the cpu prid to determine the
chip id register location and just read out the chip id.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5008/
Acked-by: John Crispin <blogic@openwrt.org>