arm: bcmbca: add bcm63148 SoC support

arm: bcmbca: add bcm63148 SoC support

BCM63148 is an Broadcom B15 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 and Broadcom 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.

Signed-off-by: William Zhang <william.zhang@broadcom.com>