sensors: Add the class device support for stk3x1x sensor

Add the sensor class device support for the stk3x1x light and
proximity sensor. By using the class device, the driver will
create the subfolder in the "/sys/class/sensors/". The userspace
app could detect the sensors by searching this folder.

Change-Id: Id601679ec219ce541e8ee92837185461c9dc74bd
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>