ax88796: Add method to take MAC from platform data

Implement a way to provide the MAC address for ax88796 devices from
their platform data. Boards might decide to set the address
programmatically, taken from boot tags or other sources.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: David S. Miller <davem@davemloft.net>