main/linux-postmarketos-allwinner: upgrade to 5.8

Supersedes !1533 (closed). We've decided to stay on released kernel versions and the pine64-kernel-5.x.y branches rather than developer trees. This MR upgrades the kernel to the latest pine64-kernel-5.8.x branch.

Please test if convergence still works as before (my monitors are not supported but some others are). The patch used to fix it in 5.7 mentions it should be fixed in 5.8, but the patch still applied cleanly so I'm not sure how much of that is true. I've re-added the patch, the DTS in 5.8 doesn't have any mention of HDMI out so I assume it's not in there yet.