meta-lxatac-bsp: arm-trusted-firmware-a: use git instead of tarball URL

[hnez]

Starting in at least late april 2024 (as indicated by this thread on the DistroKit mailing list) the old snapshot URL started responding with HTTP 401 Unauthorized errors.

Replace the tarball URL with a git URL on the same server.

[hnez]

I've assigned @Emantor now because he initially added the arm-trusted-firmware-a.inc (in the before-the-public-history times) and likely knows where it came from and where to check for upstream fixes.

[Emantor]

This came from meta-arm, my suggestion would be to pull in meta-arm as well to use arm-trusted-firmware-a.inc from the layer instead of copying and modifying it.

[hnez]

Oh my. I would have prepared the two-line solution proposed here, but if pulling in meta-arm is the cleaner solution I guess that's the way to go.
I will prepare a separate commit that pulls in meta-arm and uses the trusted-firmware-a.inc from that.

[hnez]

I've opened an issue to discuss what we want to do regarding the TF-A: #157.

Until that's decided merging this PR allows building meta-lxatac from scratch again, which is a good thing.