dt-bindings: dma: add alternative REQ/ACK protocol selection in stm32-dma

dt-bindings: dma: add alternative REQ/ACK protocol selection in stm32-dma

Default REQ/ACK protocol consists in maintaining ACK signal up to the
removal of REQuest and the transfer completion.
In case of alternative REQ/ACK protocol, ACK de-assertion does not wait the
removal of the REQuest, but only the transfer completion.
Due to a possible DMA stream lock when transferring data to/from STM32
USART/UART, this new bindings allow to select this alternative protocol in
device tree, especially for STM32 USART/UART nodes.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210624093959.142265-2-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>