iio: Add a DMAengine framework based buffer

Add a generic fully device independent DMA buffer implementation that uses
the DMAegnine framework to perform the DMA transfers. This can be used by
converter drivers that whish to provide a DMA buffer for converters that
are connected to a DMA core that implements the DMAengine API.

Apart from allocating the buffer using iio_dmaengine_buffer_alloc() and
freeing it using iio_dmaengine_buffer_free() no additional converter driver
specific code is required when using this DMA buffer implementation.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>