Add device tree based drive strength configuration support. It is needed to
pass EMI validation on our hardware.

Configuration values are based on the vendor's reference driver.

Tested on KSZ9563R.
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>