w1: slaves: w1_therm: Add temp attribute which outputs current temperature

DS18B20 and it's brothers are pretty popular in the RaspberryPi world when it comes to the temperature measurement. Each tutorial that I've found on the Internet uses the same way of parsing the output of the *<sensor>/w1_slave* sysfs file. This patch adds a dedicate file/attribute called *temp*, which only returns current temperature. Thus, the parsing is not required anymore.