hwmon: new driver for ST stts751 thermal sensor

This patch adds a HWMON driver for ST Microelectronics STTS751
temperature sensors.

Thanks-to: LABBE Corentin [for suggestions]
Thanks-to: Guenter Roeck [for suggestion and discussions]
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Cc: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>