net: phy: at803x: add MDIX support to AR8031/33

AR8035 recently gained MDIX support. The same functions will work for
the AR8031/33 PHY. We just need to add the at803x_config_aneg()
callback.

This was tested on a Kontron sl28 board.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>