iio:imu:mpu6050 add explicit mpu9250 support

The mpu9250 is a SIP containing an mpu6500 and an ak8975.  If this was all
there was too it there would be no need for explicit handling in the driver.
Arguably the bindings would also only reflect the presence of an mpu6500 with
the ak8975 hanging off it, as the kernel doesn't care that they are in one
package.

However, the WHOAMI value changes as well so best to add explicit support.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>