hwmon: (pcf8591) Register as a hwmon device

Register PCF8591 devices as hwmon devices. There's little point in
implementing the standard sysfs interface if we don't register it in
a way libsensors will pick it.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by: Guenter Roeck <guenter.roeck@ericsson.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>