dt-bindings: arm64: Add BCM4912 SoC to binding document

Add BCM4912 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>