feat(spmc/mem): add FF-A memory management code

feat(spmc/mem): add FF-A memory management code

Originally taken from the downstream Trusty SPD [1]
implementation and modified to integrate with
the EL3 SPMC internals.

Add support to the EL3 SPMC for a subset of the FF-A
memory management ABIs:
- FFA_MEM_SHARE
- FFA_MEM_LEND
- FFA_MEM_RETRIEVE_REQ
- FFA_MEM_RETRIEVE_RESP
- FFA_MEM_RELINQUISH
- FFA_MEM_RECLAIM
- FFA_MEM_FRAG_RX
- FFA_MEM_FRAG_TX

This implementation relies on a datastore allocated in
platform specific code in order to store memory descriptors
about ongoing memory transactions. This mechanism
will be implemented in the following commit.

[1] https://android.googlesource.com/trusty/external/trusted-firmware-a/+/refs/heads/master/services/spd/trusty/

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: Ib042f73c8a6e0f0aed00f6762be175cb9dedc042