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

I mentioned this in !582 (merged) already, and there is justification for this change in the commit message and in !582 (merged) ("Implementation").

From !582 (comment 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.