ethtool: (uapi) Add ETHTOOL_PHY_GTUNABLE and ETHTOOL_PHY_STUNABLE

Defines a generic API to get/set phy tunables. The API is using the
existing ethtool_tunable/tunable_type_id types which is already being used
for mac level tunables.

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>