dt-bindings: mmc: PL18x stop relying on order of dma-names

We don't care in which order are "rx" and "tx" DMA names supplied.

Fixes: 4df297aaeb ("dt-bindings: mmc: Add missing properties used in examples")
Signed-off-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20211224192545.74528-1-david@ixit.cz
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/mmc/arm,pl18x.yaml

@@ -70,9 +70,13 @@ properties:
     maxItems: 2
 
   dma-names:
-    items:
-      - const: rx
-      - const: tx
+    oneOf:
+      - items:
+          - const: tx
+          - const: rx
+      - items:
+          - const: rx
+          - const: tx
 
   power-domains: true