Add support for BCM1480 family of chips.

    
 - Kconfig and Makefile changes
 - arch/mips/sibyte/bcm1480/
 - changes to sibyte common code to support 1480
    
Signed-Off-By: Andy Isaacson <adi@broadcom.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>