net: phy: at803x: add cable diagnostics support for ATH9331 and ATH8032

Add support for Atheros 100Base-T PHYs. The only difference seems to be
the ability to test 2 pairs instead of 4 and the lack of 1000Base-T
specific register.

Only the ATH9331 was tested with this patch.

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