dt-bindings: iio: accel: Add docs for ADXL372

Add the device tree binding documentation for the ADXL372 3-axis digital
accelerometer.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>