arm: bcmbca: add bcm6855 SoC support under CONFIG_ARCH_BCMBCA

arm: bcmbca: add bcm6855 SoC support under CONFIG_ARCH_BCMBCA

BCM6855 is a Broadcom ARM A7 based PON Gateway SoC. It is part of the
BCA (Broadband Carrier Access origin) chipset family. Like other
broadband SoC, this patch adds it under CONFIG_BCM6855 chip config and
CONFIG_ARCH_BCMBCA platform config.

This initial support includes a bare-bone implementation and dts with
CPU subsystem, memory and ARM PL101 uart. This SoC is supported in the
linux-next git repository so the dts and dtsi files are copied from linux.

The u-boot image can be loaded from flash or network to the entry point
address in the memory and boot from there to the console.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>