dt-bindings: net: broadcom-bluetooth: Add BCM43540 compatible string

The BCM43540 is a 802.11 a/b/g/n/ac WiFi + Bluetooth 4.1 chip from
Broadcom. This is present in Azurewave AW-CM195NF WiFi+BT module.

Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>