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.com


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