Draft: combine linux-postmarketos-mediatek-mt8183 and -mt81xx into linux-postmarketos-mt81

This set of patches combines the -mt8183 and -mt81xx kernels into one, currently called -mt81, as both are not well maintained at the moment.

It should also be possible to combine the -mt8173 flavor, but this could also be done in a second step.

This needs thorough testing on the following devices:

• device-google-asurada
• device-google-corsola (@catto24)
• device-google-cherry
• device-google-kukui

I hope I did not miss important drivers during the config "merge", but please do a complete test like documented here

Further TODOs:

• adjust device wiki pages
• adjust soc pages
• discuss mainaintership and update CODEOWNERS

cc: @WeirdTreeThing @jenneron