net: phy: at803x: add ar9331 support

Mostly this hardware can work with generic PHY driver, but this change
is needed to provided interrupt handling support.
Tested with dsa ar9331-switch driver.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>