arm: bcmbca: add bcm6858 SoC support under CONFIG_ARCH_BCMBCA

BCM6858 is a Broadcom B53 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_BCM6858 chip config and
CONFIG_ARCH_BCMBCA platform config.

This initial support includes a bare-bone implementation and the
original dts is updated with the one from linux next git repository.

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>