dt-bindings: arm64: Add BCM6856 SoC

Add BCM6856 SoC device tree description to bcmbca binding document.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>