sensors: Add the class device support for mma8x5x sensor

Add the sensor class device support for the mma8x5x accelerometer
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: I6b96c43fa6da78d84a950f67843c62a41e03aa99
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>