linux-postmarketos-exynos5: fix Mali T628

This fixes kernel-side part of Mali T628 on Exynos 5 SoCs.

It needs patched mesa, i think of 2 ways to support it:

• small part of mesa packaged (e.g. mesa-dri-gallium-t6xx) which replaces (not provides since apk has issues with that) mesa-dri-gallium
• separated alpine repo with mesa fork which is included into device package

Anyway, with kernel-side part users can build mesa themselves and get GPU to work, so i suggest to merge this part

UPD: Predicting the question about upstreaming: it doesn't depend on me, i just ported what is done in https://forum.odroid.com/viewtopic.php?p=350810#p350810, i'm not sure about their future plans