Move linux-rpi users to downstream

linux-rpi is a downstream Linux kernel provided by the vendor and differs a lot from mainline. It duplicates mainline drivers and does not use some drivers that are upstream (like the RP1 clock driver). The diff to an upstream release is huge: For reference, the Alpine patch over the 6.12 LTS is a file with 291161 lines.

As such, devices using it do not qualify for testing. This MR moves all users of the package to downstream. The Raspberry Pi boards have very good support in mainline (except for I think the Pi 5, but I may be wrong, I'm not deep in the Pi ecosystem). If the maintainers want the packages to be in testing, community or main, they should use a close-to-mainline kernel, it should be rather straightforward for most Raspberry Pi hardware.

While I was at it, I noticed some packages depend on kernels that don't exist in Alpine anymore, I've fixed those to the best of my ability, but I cannot test that they still work.