fix(el3-spm): improve direct messaging validation

Perform additional validation of the source and destination
IDs of direct messages.
Additionally track the sender of a direct request to allow
validating the target of the corresponding direct response.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I8d39d53a02b8333246f1500c79ba04f149459c16
3 files changed