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

Commit 7d250a062f75 ("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 <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

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

···
  - 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: