SPM: RESET_TO_BL31 add a third cactus partition

A third cactus sample test partition was added to tf-a-tests.
The test fvp-spm-rst-bl31,fvp-default:fvp-spm.rstbl31 requires
the third partition to be preloaded when invoking the model as
BL1/BL2 do not exist in this test case.

Change-Id: I6bcb5b220718880cba8ec6077dd6b8877df8c22b
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
3 files changed