iio: imu: Add initial support for Bosch BMI160

BMI160 is an Inertial Measurement Unit (IMU) which provides acceleration
and angular rate measurement. It also offers a secondary I2C interface
for connecting a magnetometer sensor (usually BMM160).

Current driver offers support for accelerometer and gyroscope readings
via sysfs or via buffer interface using an external trigger (e.g.
hrtimer). Data is retrieved from IMU via I2C or SPI interface.

Datasheet is at:
	http://www.mouser.com/ds/2/783/BST-BMI160-DS000-07-786474.pdf

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