dt-bindings: iio: accel: adxl345: Add spi-3wire

Add spi-3wire because the device allows to be configured for spi 3-wire
communication.
Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240401194906.56810-5-l.rubusch@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/accel/adi,adxl345.yaml

@@ -32,6 +32,8 @@ properties:
 
   spi-cpol: true
 
+  spi-3wire: true
+
   interrupts:
     maxItems: 1