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 <paul@crapouillou.net>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://lore.kernel.org/r/20240620122726.41232-8-paul@crapouillou.net
Signed-off-by: Vinod Koul <vkoul@kernel.org>