dt-bindings: arm: add BCM6855 SoC to device tree binding document

Add BCM6855 SoC device tree description to bcmbca binding document.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>