dt-bindings: iio: dac: microchip,mcp4922: Add missing binding document

Simple binding for this family of microchip DACs.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Welling <mwelling@ieee.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-16-jic23@kernel.org