hwmon: (sht15) remove multiple driver registration

Declare an array of platform_device_id, instead of registering a driver
for each supported chip. This makes the code cleaner.
Also add a module description.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>