hwmon: (sht15) add support for the status register
* Add support for:
- Heater.
- End of battery notice.
- Ability not to reload from OTP.
- Low resolution (12bit temp, 8bit humidity).
* Add an utility function to read individual bytes from the device.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com> Acked-by: Jonathan Cameron <jic23@cam.ac.uk> Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>