bcm963158: add initial support

This add the initial support of the broadcom reference
board bcm963158 with a bcm63158 SoC.

This board has 1 GB of ram, 512 MB of flash (nand),
2 usb port, 1 uart, 4 ethernet ports (LAN), 1 ethernet port (WAN).

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>