iio: accel: adxl345: add FIFO with watermark events

iio: accel: adxl345: add FIFO with watermark events

Add a basic setup for FIFO with configurable watermark. Add a handler
for watermark interrupt events and extend the channel for the
scan_index needed for the iio channel. The sensor is configurable to use
a FIFO_BYPASSED mode or a FIFO_STREAM mode. For the FIFO_STREAM mode now
a watermark can be configured, or disabled by setting 0. Further features
require a working FIFO setup.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20241228232949.72487-4-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>