tg3: Add hwmon support for temperature

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: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>