ethtool: (uapi) Add ETHTOOL_PHY_DOWNSHIFT to PHY tunables

For operation in cabling environments that are incompatible with
1000BASE-T, PHY device may provide an automatic link speed downshift
operation. When enabled, the device automatically changes its 1000BASE-T
auto-negotiation to the next slower speed after a configured number of
failed attempts at 1000BASE-T.  This feature is useful in setting up in
networks using older cable installations that include only pairs A and B,
and not pairs C and D.

Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microsemi.com>
Signed-off-by: Allan W. Nielsen <allan.nielsen@microsemi.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>