dt-bindings: dma: ingenic: Support #dma-cells = <3>

Extend the binding to support specifying a different request type for
each direction.

Signed-off-by: Paul Cercueil <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/dma/ingenic,dma.yaml

···
     maxItems: 1
 
   "#dma-cells":
-    const: 2
+    enum: [2, 3]
     description: >
       DMA clients must use the format described in dma.txt, giving a phandle
-      to the DMA controller plus the following 2 integer cells:
+      to the DMA controller plus the following integer cells:
 
-      - Request type: The DMA request type for transfers to/from the
-        device on the allocated channel, as defined in the SoC documentation.
+      - Request type: The DMA request type specifies the device endpoint that
+        will be the source or destination of the DMA transfer.
+        If "#dma-cells" is 2, the request type is a single cell, and the
+        direction will be unidirectional (either RX or TX but not both).
+        If "#dma-cells" is 3, the request type has two cells; the first
+        one corresponds to the host to device direction (TX), the second one
+        corresponds to the device to host direction (RX). The DMA channel is
+        then bidirectional.
 
       - Channel: If set to 0xffffffff, any available channel will be allocated
         for the client. Otherwise, the exact channel specified will be used.