net: phy: bcm54140: add hwmon support

The PHY supports monitoring its die temperature as well as two analog
voltages. Add support for it.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>