test(cactus): prevent realm region access from swd

This change adds TFTF and cactus tests to check a realm region cannot
be accessed from secure world.
A non-secure buffer is delegated to realm PAS and shared to a secure
partition through FF-A memory sharing operations.
The SP retrieves the region from the SPM, maps it and attempts a write
access. The PE is expected to trigger a GPF data abort caught by a
custom exception handler.
Exception is trapped at S-EL1 within the secure partition because
Hafnium configures HCR_EL2.GPF=0 (and SCR_EL3.GPF=0).

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I8f855f394d0490b3584e60ceba4f3d2a20197495
9 files changed