arm: bcmbca: add bcm63138 SoC support

arm: bcmbca: add bcm63138 SoC support

BCM63138 is an ARM A9 based DSL Broadband 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, ARM A9 global timer
and Broadcom uart.

This SoC is supported in the linux-next git repository so the dts and
dtsi files are stripped down version of linux copies with mininum blocks
needed by u-boot.

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.

This patch applies on top of the my previous patch [1].

[1] https://lists.denx.de/pipermail/u-boot/2022-August/490570.html

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