dt-bindings: sound: sun4i-i2s: Document that the RX channel can be missing

The A83t and compatibles controllers don't have any reception capabilities
on some instances of the controllers, even though it was never documented
as such in the binding before.

Therefore, on those controllers, we don't have the option to set an RX DMA
channel.

This was already done in the DTSI, but the binding itself was never
updated. Let's add a special case in the schemas.
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/allwinner,sun4i-a10-i2s.yaml

@@ -41,19 +41,11 @@ properties:
       - const: apb
       - const: mod
 
-  dmas:
-    items:
-      - description: RX DMA Channel
-      - description: TX DMA Channel
-
-  dma-names:
-    items:
-      - const: rx
-      - const: tx
-
   # Even though it only applies to subschemas under the conditionals,
   # not listing them here will trigger a warning because of the
   # additionalsProperties set to false.
+  dmas: true
+  dma-names: true
   resets:
     maxItems: 1
 
@@ -72,6 +64,46 @@ allOf:
       required:
         - resets
 
+  - if:
+      properties:
+        compatible:
+          contains:
+            const: allwinner,sun8i-a83t-i2s
+
+    then:
+      properties:
+        dmas:
+          minItems: 1
+          maxItems: 2
+          items:
+            - description: RX DMA Channel
+            - description: TX DMA Channel
+          description:
+            Some controllers cannot receive but can only transmit
+            data. In such a case, the RX DMA channel is to be omitted.
+
+        dma-names:
+          oneOf:
+            - items:
+                - const: rx
+                - const: tx
+            - const: tx
+          description:
+            Some controllers cannot receive but can only transmit
+            data. In such a case, the RX name is to be omitted.
+
+    else:
+      properties:
+        dmas:
+          items:
+            - description: RX DMA Channel
+            - description: TX DMA Channel
+
+        dma-names:
+          items:
+            - const: rx
+            - const: tx
+
 required:
   - "#sound-dai-cells"
   - compatible