net: phy: Add downshift get/set support in Microsemi PHYs driver

Implements the phy tunable function pointers and implement downshift
functionality for MSCC PHYs.

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