net: phy: marvell: Errata for mv88e6390 internal PHYs

The VOD can be out of spec, unless some magic value is poked into an
undocumented register in an undocumented page.

Fixes: e4cf8a38

 ("net: phy: Marvell: Add mv88e6390 internal PHY")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>