iio: imu: st_lsm6dsx: add st_lsm6dsx_push_tagged_data routine

Introduce st_lsm6dsx_push_tagged_data routine to push samples
to iio buffers. st_lsm6dsx_push_tagged_data will be reused adding
hw fifo support to st_lsm6dsx i2c embedded controller in order to
improve code readability

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