dt-bindings: net: broadcom-bluetooth: Add property for autobaud mode

Add property, "brcm,requires-autobaud-mode", to enable autobaud mode
selection.

Some devices (e.g. CYW5557x) require autobaud mode to enable FW loading.
Autobaud mode can also be required on some boards where the controller
device is using a non-standard baud rate when first powered on.

Signed-off-by: Hakan Jansson <hakan.jansson@infineon.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>