iio: imu: st_lsm6dsx: add hw FIFO support to i2c controller

Introduce hw FIFO support to lsm6dsx i2c controller.
st_lsm6dsx sensor-hub relies on SLV0 for slave configuration since SLV0
is the only channel that can be used to write into i2c slave devices.
SLV{1,2,3} channels are used to read external data and push them into
the hw FIFO

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