Use the downstream trusted-firmware-a

[hnez]

The STM32MP1 downstream trusted-firmware-a has numerous benefits over the upstream TF-A [citation needed].

The discussion to use an upstream TF-A recipe came up in #156 and continued in chat.
The result of that is that we likely want to use the STM32MP1 downstream TF-A, possibly with an upstream build recipe from somewhere else.

Maybe @Emantor or @a3f can tune in to fill this discussion with some more details about the benefits of the STM32MP1 downstream TF-A, that make it worthwhile to use it.