iio: imu: st_lsm6dsx: add support for accel/gyro unit of lsm9ds1

iio: imu: st_lsm6dsx: add support for accel/gyro unit of lsm9ds1

The LSM9DS1's accelerometer / gyroscope unit and it's magnetometer (separately
supported in iio/magnetometer/st_magn*) are located on a separate i2c addresses
on the bus.

For the datasheet, see https://www.st.com/resource/en/datasheet/lsm9ds1.pdf

Treat it just like the LSM6* devices and, despite it's name, hook it up
to the st_lsm6dsx driver, using it's basic functionality.

accelerometer and gyroscope are not independently clocked. It runs at the gyro
frequencies if both are enabled, see chapter 7.12 of the datasheet.
We could have handled this as a single IIO device but we have split
it up to be more consistent with the other more flexible devices.

Despite supporting and testing the LSM9DS1, we call the gyro channels
iio_chan_spec struct "st_lsm6ds0_gyro_channels" because the register
description is equal. This suggests that supporting LSM6DS0 should be
trivial to do.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>