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>