iio: accel: fxls8962af: add threshold event handling

Add event channels that control the creation of motion events.
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20210920114221.1595543-1-sean@geanix.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>