drivers: net: cpsw: Add support for fixed-link PHY

Add support for a fixed-link devicetree sub-node in case the the
cpsw MAC is directly connected to a non-mdio PHY/device.

Signed-off-by: Markus Brunner <systemprogrammierung.brunner@gmail.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>