sensors: Add the class device support for bmp18x sensor

Add the sensor class device support for the bmp18x pressure
sensor. By using the class device, the driver will create the
subfolder in the "/sys/class/sensors/" and the userspace app
could detect the sensors by searching this folder.

Change-Id: Id947e8b7dd101d15bc7ffcbe5a01fa9c6cf28efe
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>