main/linux-postmarketos-qcom: rename to linux-postmarketos-qcom-msm8974

I mentioned this in https://gitlab.com/postmarketOS/pmaports/merge_requests/582 already, and there is justification for this change in the commit message and in https://gitlab.com/postmarketOS/pmaports/merge_requests/582#implementation ("Implementation").

From https://gitlab.com/postmarketOS/pmaports/merge_requests/582#note_210729198:

(linux-postmarketos-qcom is really more a linux-postmarketos-qcom-msm8974 at this point...)

It would be a good idea to rename it then, it would be great if you could submit a follow up MR after this one that does this change.

I tested building this locally since I suspect that CI will time out.