dt-bindings: net: bluetooth: Add device property firmware-name for QCA6174

This patch adds an optional device property "firmware-name" to allow the
driver to load customized nvm firmware file based on this property.

Signed-off-by: Rocky Liao <rjliao@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>