ASoC: dt-bindings: fsl-sai: allow only one dma-names

Some sai only connect one direction dma (rx/tx) in SOC. For example:
imx8qxp sai5 only connect tx dma channel. So allow only one "rx" or "tx"
for dma-names.

Remove description under dmas because no user use index to get dma channel.
All user use 'dma-names' to get correct dma channel. dma-names already in
'required' list.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://msgid.link/r/20240328-asrc_8qxp-v8-3-801cd6bb5be2@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/fsl,sai.yaml

@@ -81,14 +81,12 @@ properties:
 
   dmas:
     minItems: 1
-    items:
-      - description: DMA controller phandle and request line for RX
-      - description: DMA controller phandle and request line for TX
+    maxItems: 2
 
   dma-names:
     minItems: 1
     items:
-      - const: rx
+      - enum: [ rx, tx ]
       - const: tx
 
   interrupts: