support/testing: fix Marvell ATF source code

The version of the ARM Trusted Firmware from Marvell was a Git branch,
not a Git commit, leading to unreproducible results. So let's use a
Git commit instead, which is the latest available from the branch that
was previously used.

More specifically, this branch has recently seen a fix that is needed
for ATF to build properly with recent gcc versions:

  https://github.com/MarvellEmbeddedProcessors/atf-marvell/commit/c96ec59f8b819123a06e1839ab5578030cf263b0



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>