net: axienet: add support for standard phy-mode binding

Keep supporting proprietary "xlnx,phy-type" attribute and add support for
MII connectivity to the PHY.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alvaro Gamez Machado <alvaro.gamez@hazent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>