net: phy: marvell: mv88e6390 temperature sensor reading

The internal PHYs in the mv88e6390 switch have a temperature sensor.
It uses a different register layout to other PHY currently supported.
It also has an errata, in that some reads of the sensor result in bad
values. So a number of reads need to be made, and the average taken.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>