hwmon: lm75: expose to thermal fw via DT nodes

This patch adds to lm75 temperature sensor the possibility
to expose itself as thermal zone device, registered on the
thermal framework.

The thermal zone is built only if a device tree node
describing a thermal zone for this sensor is present
inside the lm75 DT node. Otherwise, the driver behavior
will be the same.

Note: This patch has also been reviewed by Jean D. He has
requested to perform a wider inspection of possible
users of thermal and hwmon interaction API. On the other
hand, the change on this patch is acceptable on first
step of overall code change.

Cc: Jean Delvare <khali@linux-fr.org>
Cc: lm-sensors@lm-sensors.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>