net: davinci_emac: Add support for fixed-link PHY

In case the DaVinci Emac 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: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David S. Miller <davem@davemloft.net>