ixp4xx_eth: Set MAC address from device tree

If there is a MAC address specified in the device tree, then
use it. This is already perfectly legal to specify in accordance
with the generic ethernet-controller.yaml schema.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>