iio: buffer-dma: add iio_dmaengine_buffer_setup()

iio: buffer-dma: add iio_dmaengine_buffer_setup()

This brings the DMA buffer API more in line with what we have in the
triggered buffer. There's no need of having both
devm_iio_dmaengine_buffer_setup() and devm_iio_dmaengine_buffer_alloc().
Hence we introduce the new iio_dmaengine_buffer_setup() that together
with devm_iio_dmaengine_buffer_setup() should be all we need.

Note that as part of this change iio_dmaengine_buffer_alloc() is again
static and the axi-adc was updated accordingly.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240419-iio-backend-axi-dac-v4-1-5ca45b4de294@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>