The 4373 has not been formally released to the broad market, but we do have module partners working to develop certified modules based on the 4373 (the first will be released in Q3). I'd recommend that you reach out to Laird Connectivity and Murata in your region. They can provide guidance on the backporting process as well.
The FMAC driver has been provided.
As described in the README, CYW4373 is supported.
"Backports package" is available from older kernel to 5.4.18(or 4.14.77) kernel.
"fmac patch files" is for specified kernel version, so I think it will need to be modified to apply to different versions.