dt-bindings: iio: accel: Add DT binding doc for ADXL355

Add devicetree binding document for ADXL355, a 3-Axis MEMS Accelerometer.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210811073027.124619-2-puranjay12@gmail.com


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