create a branch for mainline linux drivers

@dorota.czaplejewicz @sebastian.krzyszkowiak I just want to share what I'm starting to work on, because I haven't worked on the code much yet. My goal is to do a branch based on the latest gtk3 upstream tag, that includes some but not all of our downstream stuff:

• everything CI/debian related we have now
• everything related to 10bit handling and other minor image-improvements
• none of the changes related to the "legacy" driver handling we added
• (if not yet supported, handling multiple /dev/media devices because the pinephone has only one, with 2 sensor subdevs)

what I hope to be able to run this megapixels on, is Linux without the nxp out-of-tree drivers. MR: https://source.puri.sm/Librem5/linux-next/-/merge_requests/418