net: phy: tja11xx: add SQI support

This patch implements reading of the Signal Quality Index for better
cable/link troubleshooting.

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>