net: phy: marvell: support downshift as PHY tunable

So far downshift is implemented for one small use case only and can't
be controlled from userspace. So let's implement this feature properly
as a PHY tunable so that it can be controlled via ethtool.
More Marvell PHY's may support downshift, but I restricted it for now
to the ones where I have the datasheet.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>