Some tg3 devices have management firmware that can export sensor data.
Export temperature sensor reading via hwmon sysfs.

[hwmon interface suggested by Ben Hutchings <bhutchings@solarflare.com>]
Signed-off-by: NMatt Carlson <mcarlson@broadcom.com>
Signed-off-by: NNithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: NMichael Chan <mchan@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>