ARM: dts: Add DTS files for bcmbca SoC BCM6878

Add dts for ARMv7 based broadband SoC BCM6878. bcm6878.dtsi is the
SoC description dts header and bcm96878.dts is a simple dts file for
Broadcom BCM96878 Reference board that only enable the UART port.

Signed-off-by: Anand Gore <anand.gore@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>