iio: dac: mcp47a1: add support for new device

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

Signed-off-by: Joshua Crofts <joshua.crofts1@gmail.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>