support/testing: switch TestATFAllwinner to mainline TF-A

Switch to mainline TF-A that provides basic support for H5 and A64.

Note that Allwinner platform layer in TF-A does not provide support
for GCC stack protection, so make sure to disable this TF-A feature.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>