dt-bindings: iio: dac: add support for mcp47a1

The Microchip MCP47A1 is a 6-bit volatile Digital-to-Analog converter
which communicates via I2C.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>