arm: bcmbca: add bcm6756 SoC support

arm: bcmbca: add bcm6756 SoC support

BCM6756 is an ARM A7 based WLAN Gateway and Access Point 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 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.

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