MIPS: BCM63XX: Use the Chip ID register for identifying the SoC

Newer BCM63XX SoCs use virtually the same CPU ID, differing only in the
revision bits. But since they all have the Chip ID register at the same
location, we can use that to identify the SoC we are running on.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/3955/


Reviewed-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>