fix(el3-spmc): fix detection of overlapping memory regions

The current logic does not cover all scenarios of overlapping
memory regions. Update the implementation to verify non-overlapping
regions instead. Reported by Matt Oh, Google Android Red Team.

Reported-by:  <mattoh@google.com>
Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I16c53d081e4455bc0e28399d28a1b27b1a9eb49c