feat(spmc/mem): support multiple endpoints in memory transactions

Enable FFA_MEM_LEND and FFA_MEM_SHARE transactions to support multiple
borrowers and add the appropriate validation. Since we currently
only support a single S-EL1 partition, this functionality is to
support the use case where a VM shares or lends memory to one or
more VMs in the normal world as part of the same transaction to
the SP.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: Ia12c4357e9d015cb5f9b38e518b7a25b1ea2e30e