net: davinci_emac: Get device MAC on 3517

Looks like on 3517 davinci_emac MAC address registers have a
different layout compared to dm816x and am33xx.

Let's add a function to get the 3517 MAC address.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>