sensor: adxl345: Allow fifo-watermark configurable through dts

Allow for users to define the fifo-watermark on a per-instance basis
through device-tree properties. This setting is validated at build
time, so missing it when required, or setting an invalid value should
not end up in a run-time errror (as in: it runs but nothing happens).

Signed-off-by: Luis Ubieda <luisf@croxel.com>