dt-bindings: net: bluetooth: Add device tree bindings for QTI chip WCN3991

Add compatible string for the Qualcomm WCN3991 Bluetooth controller

Signed-off-by: Venkata Lakshmi Narayana Gubba <gubbaven@codeaurora.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>