iio: accel: adxl345: add single tap feature

iio: accel: adxl345: add single tap feature

Add the single tap feature with a threshold in 62.5mg/LSB points and a
scaled duration in us. Keep singletap threshold in regmap cache but
the scaled value of duration in us as member variable.

Both use IIO channels for individual enable of the x/y/z axis. Initializes
threshold and duration with reasonable content. When an interrupt is
caught it will be pushed to the according IIO channel.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250414184245.100280-3-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>