net: usb: asix: ax88772: add phylib support

To be able to use ax88772 with external PHYs and use advantage of
existing PHY drivers, we need to port at least ax88772 part of asix
driver to the phylib framework.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>