net: macb: add fixed-link node support

In case the MACB is directly connected to a
non-mdio PHY/device, it should be possible to provide
a fixed link configuration in the DT.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>