arm: bcmbca: add bcm4908 SoC support

arm: bcmbca: add bcm4908 SoC support

BCM4908 is a Broadcom B53 based 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 Broadcom uart.

This SoC is supported in the linux 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.

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