hwmon: (tmp401) Add documentation

Documentation for the tmp401 driver.

The documentation describes the tmp401 driver and the supported Texas
Instruments TMP401 and TMP411 temperature sensor chips.

Further documentation for new sysfs attributes supported by this
driver is added to Documentation/hwmon/sysfs-interface.

Signed-off-by: Andre Prendel <andre.prendel@gmx.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>