dt-bindings: iio: dac: add MCP4821

Adds support for MCP48xx series of DACs.

Datasheet: https://ww1.microchip.com/downloads/en/DeviceDoc/22244B.pdf #MCP48x1
Datasheet: https://ww1.microchip.com/downloads/en/DeviceDoc/20002249B.pdf

 #MCP48x2
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Anshul Dalal <anshulusr@gmail.com>
Link: https://lore.kernel.org/r/20231220151954.154595-1-anshulusr@gmail.com


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