iio: imu: Add mpu9255 support to mpu6050 driver

Added support for the mpu9255 IMU to the mpu6050 driver. The
register map is the same as the other chips; the only driver
difference is the compatible string and the WHOAMI register
value.

Signed-off-by: Douglas Fischer <fischerdouglasc@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>