sensors: Add the class device support for mpu3050 sensor

Add the sensor class device support for the mpu3050 gyroscope
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: Ieba9cebaa2bb62da223d78290b64865142c945f7
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>