iio: st_sensors: Add lsm9ds0 IMU support

iio: st_sensors: Add lsm9ds0 IMU support

We can utilize separate drivers for accelerometer and magnetometer,
so here is the glue driver to enable LSM9DS0 IMU support.

The idea was suggested by Crestez Dan Leonard in [1]. The proposed change
was sent as RFC due to race condition concerns, which are indeed possible.

In order to amend the initial change, I went further by providing a specific
multi-instantiate probe driver that reuses existing accelerometer and
magnetometer.

[1]: https://lore.kernel.org/patchwork/patch/670353/

Suggested-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Cc: mr.lahorde@laposte.net
Cc: Matija Podravec <matija_podravec@fastmail.fm>
Cc: Sergey Borishchenko <borischenko.sergey@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210414195454.84183-6-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>