arm: bcmbca: add bcm6813 SoC support

BCM6813 is a Broadcom B53 based PON and WLAN AP router SoC. It is part
of the BCA (Broadband Carrier Access origin) chipset family so it's
added under ARCH_BCMBCA platform. This initial support includes a
bare-bone implementation and dts with CPU subsystem, memory and ARM
PL011 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: Florian Fainelli <f.fainelli@gmail.com>