Hello @Werner,
you have been right. The patch was not included even if I updated the repo which was strange. So started over, deleted everything, cloned it and compiled it with with:
sudo ./compile.sh BOARD=espressobin BRANCH=current RELEASE=focal BUILD_MINIMAL=no BUILD_DESKTOP=no KERNEL_ONLY=no KERNEL_CONFIGURE=no COMPRESS_OUTPUTIMAGE=sha,gpg,img
This time I could find the patch in "cache/sources/linux-mainline/linux-5.10.y/drivers/pci/quirks.c" and guess what. I could use the SATA-Controller card with all HDDs connected and no kernel panic appeared!
So everything is working. Thank you a lot!
But I guess this bug is now only patched for espressobin, right? Basically it's a kernel bug which affects nearly all SBCs offering mPCI-E slots, when using SATA-Controller cards. Since AS-Media is the only vendor I could find offering such chips it might be good to have this patch in all board patch-sets until the latest linux kernel where the bug has been fixed lands in Armbian base.