Finally sorted out with my changes.
So the major headaches for this device now are GPU and cameras. Mesa driver still left undeveloped and Tegra's camera interface is not documented. I don't know how the camera support is made here. Tegra VI (Video Interface), which appears to be camera controller, is undocumented, however, it may be unused - when I open camera app in Android, I can see (in dmesg) how userspace camera driver uploads something to AVP (Audio-Video Processor, actually a crypto thing, but that's different story), and maybe that's just a video encoder program. I can't see any register access inside kernel for camera interface. That's very strange.
USB HSIC mode, required for modem to work, is possible to implement though.
S6E63M0 panel driver still not mainlined - I think it's lost because other panel drivers appeared in the repo, but no S6E63M0. It was accepted on the mailing list - see https://lkml.org/lkml/2019/4/23/366