net: phy: at803x: add mdix configuration support for AR9331 and AR8035

This patch add MDIX configuration ability for AR9331 and AR8035. Theoretically
it should work on other Atheros PHYs, but I was able to test only this
two.

Since I have no certified reference HW able to detect or configure MDIX, this
functionality was confirmed by oscilloscope.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>