iio: dac: add support for AXI DAC IP core

Support the Analog Devices Generic AXI DAC IP core. The IP core is used
for interfacing with digital-to-analog (DAC) converters that require either
a high-speed serial interface (JESD204B/C) or a source synchronous parallel
interface (LVDS/CMOS). Typically (for such devices) SPI will be used for
configuration only, while this IP core handles the streaming of data into
memory via DMA.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240419-iio-backend-axi-dac-v4-9-5ca45b4de294@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>