net: ag71xx: add ethtool support

Add basic ethtool support. The functionality was tested on AR9331 SoC.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>