iio:imu:mpu6050: icm20608 initial support

Introduce support for Invense ICM20608 IMU, a 6-axis motion tracking device
that combines a 3-axis gyroscope and a 3-axis accelerometer:
http://www.invensense.com/products/motion-tracking/6-axis/icm-20608-2



Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>