main/linux-postmarketos-*: upgrade mainline and stable kernel to the latest from kernel.org

I figured that the kernel are a little bit dated, so I just upgraded it to the latest.

At the time making this MR, kernel.org reports the 5.1 kernel as mainline, and 5.1.2 as stable.

P.S: I haven't upgraded linux-postmarketos-qcom due to backlight patch failing concern, and linux-postmarketos-allwinner was for working on the upcoming PinePhone.

Anyone here working on lg-hammerhead should look at this and try to patch it if you guys need the qcom kernel to be upstreamed. 😉

Also GitLab CI will never pass when building because it'll exceed the 1 hour mark.