dt-bindings: iio: accel: Add binding documentation for ADXL313

Add device tree binding documentation for ADXL313 3-axis accelerometer.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/2eff22d1d22f7e72efdabdc681d02e922682c434.1630523106.git.lucas.p.stankus@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>