dt-bindings: iio: accel: Document ADXL345 accelerometer binding

Add the device tree binding documentation for the ADXL345 3-axis digital
accelerometer.

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>