dt-bindings: net: document Bluetooth bindings in one place

In the same way as Ethernet, gather the Bluetooth related bindings in
one file. Introduce the bluetooth-bd-address property which can be used
to store the assigned BD address.
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

Documentation/devicetree/bindings/net/bluetooth.txt

+The following properties are common to the Bluetooth controllers:
+
+- local-bd-address: array of 6 bytes, specifies the BD address that was
+  uniquely assigned to the Bluetooth device, formatted with least significant
+  byte first (little-endian).