feat(spmc/mem): prevent duplicated sharing of memory regions

Allow the SPMC to reject incoming memory sharing/lending requests
that contain memory regions which overlap with an existing
request.

To enable this functionality the SPMC compares each requested
memory region to those in ongoing memory transactions and rejects
the request if the ranges overlap.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I7588846f272ec2add2a341d9f24836c73a046e2f