iio: inv_mpu6050: Add explicit support for MPU9150

This device is a package containing a MPU6050-like sensor and an AK8975
magnetometer. The magnetometer component is supported by the existing
ak8975 driver.

This patch also rephrases the Kconfig descriptions.

Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Acked-by: Ge Gao <ggao@invensense.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>