net: atlantic: implement phy downshift feature

PHY downshift allows phy to try renegotiate if link is unstable
and can carry higher speed.

AQC devices has integrated PHY which is controlled by MAC firmware.
Thus, driver defines new ethtool callbacks to implement phy tunables
via netdev.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>