Reintroduce Cactus-MM and SPM-MM tests

The code has been taken from commit 99f4fd28 ("cactus: Use UART2
instead of UART0") and modified slightly to be integrated in the current
master.

There are three tests that are failing in the CI. They have been
disabled for the time being:

- mem_attr_changes_tests() in cactus_main() in the file
  spm/cactus_mm/cactus_mm_main.c.

- Two tests in the file tftf/tests/tests-spm-mm.xml.

Change-Id: I6332cbff1cefeb82b9447fae1b613879e65186a1
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>