iio: accel: kxsd9: Add device tree bindings

This accelerometer can be probed from the device tree, so it needs
to have proper documentation of it's device tree bindings.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>