dt-bindings: net: bluetooth: Add compatible string for WCN3991

Commit 7d250a06 ("Bluetooth: hci_qca: Add support for Qualcomm
Bluetooth SoC WCN3991") added the compatible string 'qcom,wcn3991-bt'
to the Qualcomm Bluetooth driver, however the string is not listed
in the binding. Add the 'qcom,wcn3991-bt' to the supported compatible
strings.
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/net/qualcomm-bluetooth.txt

@@ -11,6 +11,7 @@ Required properties:
  - compatible: should contain one of the following:
    * "qcom,qca6174-bt"
    * "qcom,wcn3990-bt"
+   * "qcom,wcn3991-bt"
    * "qcom,wcn3998-bt"
 
 Optional properties for compatible string qcom,qca6174-bt: