linux-nokia-n900: upgrade kernel to 5.15.12
Happy New Year 2022!
This merge request aims to upgrade the N900 kernel to 5.15.12 (the latest stable LTS kernel).
Patches in this merge request were taken from Maemo Leste while they were doing some rebasing, those are required as the system won't be stable without them (especially 0004 and 0007).