dt-bindings: dma: add direct mode support through device tree in stm32-dma

Direct mode or FIFO mode is computed by stm32-dma driver. Add a way for the
user to force direct mode, by setting bit 2 in the bitfield value
specifying DMA features in the device tree.
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20200422102904.1448-2-amelie.delaunay@st.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/devicetree/bindings/dma/st,stm32-dma.yaml

@@ -36,6 +36,11 @@ description: |
          0x1: 1/2 full FIFO
          0x2: 3/4 full FIFO
          0x3: full FIFO
+       -bit 2: DMA direct mode
+         0x0: FIFO mode with threshold selectable with bit 0-1
+         0x1: Direct mode: each DMA request immediately initiates a transfer
+              from/to the memory, FIFO is bypassed.
+
 
 maintainers:
   - Amelie Delaunay <amelie.delaunay@st.com>