iio: imu: st_lsm6dsx: add i2c embedded controller support

i2c controller embedded in lsm6dx series can connect up to four
slave devices using accelerometer sensor as trigger for i2c
read/write operations.
Introduce sensor hub support for lsm6dso sensor. Add register map
for lis2mdl magnetometer sensor.
In order to perform single read/write operations st_lsm6dsx driver
relies on SLV0 channel (hw FIFO is not supported yet)

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>