arm64: dts: Add DTS files for bcmbca SoC BCM6813

Add DTS for ARMv8 based broadband SoC BCM6813. bcm6813.dtsi is the
SoC description DTS header and bcm96813.dts is a simple DTS file for
Broadcom BCM96813 Reference board that only enable the UART port.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>