feat(arm): forbid running RME-enlightened BL31 from DRAM
According to Arm CCA security model ,
"Root world firmware, including Monitor, is the most trusted CCA
component on application PE. It enforces CCA security guarantees for
not just Realm world, but also for Secure world and for itself.
It is expected to be small enough to feasibly fit in on-chip memory,
and typically needs to be available early in the boot process when
only on-chip memory is available."
For these reasons, it is expected that "monitor code executes entirely
from on-chip memory."
This precludes usage of ARM_BL31_IN_DRAM for RME-enlightened firmware.
 Arm DEN0096 A.a, section 7.3 "Use of external memory by CCA".
Signed-off-by: Sandrine Bailleux <firstname.lastname@example.org>
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