dt-bindings: iio: adc: add lltc,ltc2309 bindings

Add devicetree bindings for the Linear Technology LTC2309 ADC driver.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Link: https://lore.kernel.org/r/20230828-ltc2309-v3-1-338b3a8fab8b@gmail.com


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