[MIPS] Mention Broadcom part number for BigSur board

    
Mention the Broadcom part number for the BigSur board (BCM91480B)
in Kconfig, just like it's done for other Broadcom boards.
    
Signed-off-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>