dt-bindings: arm: add bcmbca device tree binding document

Add arch bcmbca device tree binding document for Broadcom ARM based
broadband SoC chipsets. In this change, only BCM47622 is added. Other
chipsets will be added in the future.

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>