net: ethernet: mvpp2: Implement ethtool::nway_reset

Implement ethtool::nway_reset using phy_ethtool_nway_reset. We are
already using dev->phydev all over the place so this comes for free.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>