Documentation: dmaengine: Document new dma_vec API

Document the dmaengine_prep_peripheral_dma_vec() API function, the
device_prep_peripheral_dma_vec() backend function, and the dma_vec
struct.

Signed-off-by: Paul Cercueil <[e-mail]>
Reviewed-by: Bagas Sanjaya <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/driver-api/dmaengine/client.rst

···
 
   - slave_sg: DMA a list of scatter gather buffers from/to a peripheral
 
+  - peripheral_dma_vec: DMA an array of scatter gather buffers from/to a
+    peripheral. Similar to slave_sg, but uses an array of dma_vec
+    structures instead of a scatterlist.
+
   - dma_cyclic: Perform a cyclic DMA operation from/to a peripheral till the
     operation is explicitly stopped.
 
···
      struct dma_async_tx_descriptor *dmaengine_prep_slave_sg(
 		struct dma_chan *chan, struct scatterlist *sgl,
 		unsigned int sg_len, enum dma_data_direction direction,
+		unsigned long flags);
+
+     struct dma_async_tx_descriptor *dmaengine_prep_peripheral_dma_vec(
+		struct dma_chan *chan, const struct dma_vec *vecs,
+		size_t nents, enum dma_data_direction direction,
 		unsigned long flags);
 
      struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(

Documentation/driver-api/dmaengine/provider.rst

···
     - residue: Provides the residue bytes of the transfer for those that
       support residue.
 
+- ``device_prep_peripheral_dma_vec``
+
+  - Similar to ``device_prep_slave_sg``, but it takes a pointer to a
+    array of ``dma_vec`` structures, which (in the long run) will replace
+    scatterlists.
+
 - ``device_issue_pending``
 
   - Takes the first transaction descriptor in the pending queue,
···
 
 - Not really relevant any more since the introduction of ``virt-dma``
   that abstracts it away.
+
+dma_vec
+
+- A small structure that contains a DMA address and length.
 
 DMA_CTRL_ACK