iio: buffer-dmaengine: export buffer alloc and free functions

Export iio_dmaengine_buffer_free() and iio_dmaengine_buffer_alloc().
This is in preparation of introducing IIO backends support. This will
allow us to allocate a buffer and control it's lifetime from a device
different from the one holding the DMA firmware properties. Effectively,
in this case the struct device holding the firmware information about
the DMA channels is not the same as iio_dev->dev.parent (typical case).

While at it, namespace the buffer-dmaengine exports and update the
current user of these buffers.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240210-iio-backend-v11-4-f5242a5fb42a@analog.com


Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>