dt-bindings: sound: gtm601: Add description

This allows to us to document the channel and sampling
rate requirements.

Signed-off-by: Guido Günther <[e-mail]>
Reviewed-by: David Heidelberg <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/option,gtm601.yaml

···
 properties:
   compatible:
     oneOf:
-      - items:  # 48 kHz stereo
+      - description: Broadmobi BM818 (48Khz stereo)
+        items:
           - const: broadmobi,bm818
           - const: option,gtm601
-      - const: option,gtm601  # 8 kHz mono
+      - description: GTM601 (8kHz mono)
+        const: option,gtm601
 
   '#sound-dai-cells':
     const: 0