net: ag71xx: port to phylink

The port to phylink was done as close as possible to initial
functionality.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>