dt-bindings: Add documentation for Broadcom Hurricane 2 SoCs

Add binding documentation for the Broadcom Hurricane 2 SoCs used in
switching control planes.

Acked-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>