feat(spmc): add function to determine the return path from the SPMC

Use knowledge of the target partition ID and source security state
to determine which route should be used to exit the SPMC.

There are 3 exit paths:
1) Return to the normal world via the SPMD, this will take care of
   switching contexts if required.
2) Return to the secure world when the call originated in the normal
   world and therefore switch contexts.
3) Return to the secure world when the call originated in the secure
   world, therefore we can return directly.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I4037f3a8a8519e2c9f1876be92806d2c41d0d154
1 file changed