Merge branch 'phy-marvell-support-downshift-as-PHY-tunable'

Heiner Kallweit says:

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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.
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Signed-off-by: David S. Miller <davem@davemloft.net>