dmaengine: Add documentation for new memcpy scatter-gather function

Documentation describes semantics, limitations and a typical use case
scenario.

Signed-off-by: Adrian Larumbe <adrianml@alumnos.upm.es>
Link: https://lore.kernel.org/r/20211101180825.241048-2-adrianml@alumnos.upm.es


Signed-off-by: Vinod Koul <vkoul@kernel.org>