MIPS: BMIPS: Add support for BCM63268

MIPS: BMIPS: Add support for BCM63268

This SoC is very similar to BCM63168 and Broadcom usually refers to them as
BCM63268.
Add BCM63268 and missing BCM63168 to device tree documentation.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: f.fainelli@gmail.com
Cc: jogo@openwrt.org
Cc: cernekee@gmail.com
Cc: robh@kernel.org
Cc: simon@fire.lp0.eu
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13042/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>