fix(spmc): fix relinquish validation check

The current implementation expects that the endpoint IDs of all
participants of a memory transaction to be listed in the relinquish
descriptor. As per the FF-A spec, aside from the current partition
ID, only the IDs of stream endpoints whose behalf it is relinquishing
the memory region must be specified.

The current implementation does not currently support proxy endpoints
therefore ensure that the endpoint count is always equal to 1 and
no stream endpoint IDs are specified and instead just verify the
caller is a valid participant in the memory transaction.

Additionally reuse the updated check in the retrieve request flow
for additional verification.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I3b970196af8a16b2531607775398cb8a2473793b