arm64: dts: Add DTS files for bcmbca SoC BCM4912

Add DTS for ARMv8 based broadband SoC BCM4912. bcm4912.dtsi is the
SoC description DTS header and bcm94912.dts is a simple DTS file for
Broadcom BCM94912 Reference board that only enable the UART port.

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>