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>
(cherry picked from commit 28517c02)