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

Documentation/devicetree/bindings/iio/adc/lltc,ltc2497.yaml

@@ -4,12 +4,21 @@
 $id: http://devicetree.org/schemas/iio/adc/lltc,ltc2497.yaml#
 $schema: http://devicetree.org/meta-schemas/core.yaml#
 
-title: Linear Technology / Analog Devices LTC2497 ADC
+title: Linear Technology / Analog Devices LTC2497 and LTC2309 ADC
 
 maintainers:
   - Michael Hennerich <michael.hennerich@analog.com>
+  - Liam Beguin <liambeguin@gmail.com>
 
 description: |
+  LTC2309:
+    low noise, low power, 8-channel, 12-bit successive approximation ADC with an
+    I2C compatible serial interface.
+
+    https://www.analog.com/media/en/technical-documentation/data-sheets/2309fd.pdf
+
+  LTC2497:
+  LTC2499:
     16bit ADC supporting up to 16 single ended or 8 differential inputs.
     I2C interface.
 
@@ -19,6 +28,7 @@ description: |
 properties:
   compatible:
     enum:
+      - lltc,ltc2309
       - lltc,ltc2497
       - lltc,ltc2499