dt-bindings: iio: adc: ti,ads1015: add compatible for tla2021

TI tla2021 is a limited single-channel variant of tla2024 which is
similar enough to be easily supportable through the same driver.

Add compatible string for tla2021 so boards may describe it in
device-tree.
Signed-off-by: Josua Mayer <josua@solid-run.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240602-cn9130-som-v6-3-89393e86d4c7@solid-run.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/adc/ti,ads1015.yaml

@@ -18,6 +18,7 @@ properties:
     enum:
       - ti,ads1015
       - ti,ads1115
+      - ti,tla2021
       - ti,tla2024
 
   reg: