dt-bindings: sound: gtm601: add the broadmobi interface

The Broadmobi BM818 uses a different sample rate and channels from the
option modem.
Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191223154712.18581-3-angus@akkea.caSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/gtm601.txt

 GTM601 UMTS modem audio interface CODEC
 
-This device has no configuration interface. Sample rate is fixed - 8kHz.
+This device has no configuration interface. The sample rate and channels are
+based on the compatible string
+	"option,gtm601" = 8kHz mono
+	"broadmobi,bm818" = 48KHz stereo
 
 Required properties:
 
-  - compatible : "option,gtm601"
+  - compatible : one of
+	"option,gtm601"
+	"broadmobi,bm818"
+
 
 Example: