iio: add MCP4728 I2C DAC driver

MCP4728 is a 12-bit quad channel DAC with I2C interface.

support for:
 * per-channel gain
 * per-channel power state
 * per-channel power down mode control
 * per-channel vref selection internal/vdd
 * store current state to on-chip EEPROM

Signed-off-by: Andrea Collamati <andrea.collamati@gmail.com>
Link: https://lore.kernel.org/r/a0933003ed3c855f9d80d6ce0a40add2b6f0ba36.1691066050.git.andrea.collamati@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>