iio: imu: Add support for Kionix KMX61 sensor

Minimal implementation for KMX61 6-axis accelerometer/magnetometer. It exports
raw accel/magn readings together with scale and sampling frequency.

Datasheet will be available at:
http://www.kionix.com/6-axis-accelerometer-magnetometer/kmx61



Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>