iio: imu: add Bosch Sensortec BNO055 core driver

Add the core driver for the BNO055 IMU from Bosch. This IMU can be
connected via both serial and I2C busses; separate patches will add support
for them.

The driver supports "AMG" (Accelerometer, Magnetometer, Gyroscope) mode,
that provides raw data from the said internal sensors, and a couple of
"fusion" modes (i.e. the IMU also does calculations in order to provide
euler angles, quaternions, linear acceleration and gravity measurements).

In fusion modes the AMG data is still available (with some calibration
refinements done by the IMU), but certain settings such as low pass filters
cut-off frequency and sensors' ranges are fixed, while in AMG mode they can
be customized; this is why AMG mode can still be interesting.

Signed-off-by: Andrea Merello <andrea.merello@iit.it>
Link: https://lore.kernel.org/r/20220907132205.28021-9-andrea.merello@iit.it
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>