iio: imu: st_lsm6dsx: add tap event detection

iio: imu: st_lsm6dsx: add tap event detection

In order to allow sensors to advertise tap event capability and report tap
events, define a new struct iio_event_spec array that includes a tap event
spec, and a new struct iio_chan_spec array that references the new
iio_event_spec array; for the LSM6DSV chip family, use the new
iio_chan_spec array and define an event source for tap events.
Tested on LSMDSV16X.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>