net: ethernet: add ag71xx driver

Add support for Atheros/QCA AR7XXX/AR9XXX/QCA95XX built-in ethernet mac support

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