ARM: dts: Add DTS files for bcmbca SoC BCM6756

Add DTS for ARMv7 based broadband SoC BCM6756. bcm6756.dtsi is the SoC
description DTS header and bcm96756.dts is a simple DTS file for
Broadcom BCM96756 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>