Bluetooth: btnxpuart: Add support for 4M baudrate

This adds support for 4000000 as secondary baudrate.

This value is selected from device tree property "max-speed"
which is then used to download FW chunks, and as operational baudrate after
HCI initialization is done.

Earlier, the secondary baudrate was fixed to 3000000 in driver, but now
with "max-speed" property, this secondary baudrate can be set to
4000000.

The secondary baudrate is set by the driver by sending a vendor command
(3F 09) to the firmware, with secondary baudrate parameter, in
nxp_post_init().

Any other value set for max-speed other than 3000000 or 4000000 will
default to 3000000, which is supported by all legacy and new NXP chipsets.

This feature is applicable for all new V3 bootloader chips and w8987 V1
bootloader chip.

This property does not apply for w8997 compatible device, since it
downloads a helper.bin FW file that sets secondary baudrate as 3000000
only.

The switch to 4000000 baudrate is validated using a Saleae Logic Analyzer
and imx8m-mini with AW693 M.2 module.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>