net: phy: marvell: add PHY tunable support for more PHY versions

More PHY versions are compatible with the existing downshift
implementation, so let's add downshift support for them.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>