ARM: dts: Add DTS files for bcmbca SoC BCM6855

Add DTS for ARMv7 based broadband SoC BCM6855. bcm6855.dtsi is the SoC
description DTS header and bcm96855.dts is a simple DTS file for
Broadcom BCM96855 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>