Add support for fixed-links in configurations without PHY.
(e.g. connection to a switch, SGMII point to point, SFPs)

Check: Documentation/devicetree/bindings/net/fixed-link.txt.
Signed-off-by: Andreas Oetken <ennoerlangen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>