dt-bindings: iio: dac: add mcp4728.yaml

Add documentation for MCP4728

Signed-off-by: Andrea Collamati <andrea.collamati@gmail.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/d93dd116cfa7f958c038c0c62993071ea48451d2.1691066050.git.andrea.collamati@gmail.com


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