Draft: main/arm-trusted-firmware: build separate bl31 with regulator setup disabled

This is needed to get Ethernet working on Orange Pi 3.

My previous effort was !24978 (closed), but since this commit in ATF it's possible to make a much cleaner workaround.

Draft until next upstream release.

It will also require to do a small path correction in u-boot APKBUILD, but it should be in another MR, I guess.