Documentation: dmaengine: Document new dma_vec API

  - 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.

		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(

		struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,

		size_t period_len, enum dma_data_direction direction);

    - 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

- If clear, the descriptor cannot be reused by provider until the