iio: imu: inv_mpu6050: Add SPI support for MPU6000

The only difference between the MPU6000 and the
MPU6050 is that the first also supports SPI.
Add SPI driver for this chip.

Signed-off-by: Adriana Reus <adriana.reus@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>