ASoC: dt-bindings: simple-card: Document simple-audio-card,plat

The simple card driver already has support for a simple-audio-card,plat
property but its not reflected in the documentation. Add documentation
for this plat property.
Signed-off-by: Faiz Abbas <faiz.abbas@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230117061808.18422-1-faiz.abbas@arm.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/simple-card.yaml

@@ -205,6 +205,8 @@ patternProperties:
     $ref: "#/definitions/dai"
   "^simple-audio-card,codec(@[0-9a-f]+)?$":
     $ref: "#/definitions/dai"
+  "^simple-audio-card,plat(@[0-9a-f]+)?$":
+    $ref: "#/definitions/dai"
 
   "^simple-audio-card,dai-link(@[0-9a-f]+)?$":
     description: |