net: ethernet: bgmac: Allow MAC address to be specified in DTB

Allows the BCMA version of the bgmac driver to obtain MAC address
from the device tree.  If no MAC address is specified there, then
the previous behavior (obtaining MAC address from SPROM) is
used.

Signed-off-by: Steve Lin <steven.lin1@broadcom.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>