iio: dac: mcp4922: add support to mcp4921

Add support to mcp4921 which has only one output channel.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220624095619.1415614-1-kory.maincent@bootlin.com


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