arm64: dts: apq8096-db820c: enable bluetooth node

Add a new serial node for the Qualcomm BT controller QCA6174. This
allows automatic probing and hci registration through the serdev
framework instead of relying on the userspace helpers.

Signed-off-by: Thierry Escande <thierry.escande@linaro.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>