dt-bindings: iio: dac: mcp4922: expand for mcp4921 support

Add device tree bindings for the MCP4921 DAC.
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220624095619.1415614-2-kory.maincent@bootlin.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/microchip,mcp4922.yaml

@@ -15,6 +15,7 @@ properties:
     enum:
       - microchip,mcp4902
       - microchip,mcp4912
+      - microchip,mcp4921
       - microchip,mcp4922
 
   reg: