dt-bindings: net: bluetooth: nxp: Add support for 4M baudrate

Add support for 4000000 as secondary baudrate for NXP chipsets
supporting max baudrate as 4M, and are close to the host processor on
same PCB. This mainly helps with faster FW download.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>