phy: marvell: Add support for temperature sensor

Some Marvell PHYs have an inbuilt temperature sensor. Add hwmon
support for this sensor.

There are two different variants. The simpler, older chips have a 5
degree accuracy. The newer devices have 1 degree accuracy.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>