dt-bindings: iio: accel: adxl345: switch to YAML bindings

The ADX345 supports both I2C & SPI bindings.
This change switches from old text bindings, to YAML bindings, and also
tries to make use of the recent multiple-examples support.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>