feat(spmc): add support for FFA_MSG_WAIT

Handle an incoming call of FFA_MSG_WAIT from the secure world
and update the runtime state of the calling partition accordingly.

This ABI can be called in the following scenarios:
  - Used by an SP to signal it has finished initializing.
  - To resume the normal world after handling a secure interrupt
    that interrupted the normal world.
  - To relinquish control back to the normal world.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I929713a2280e8ec291b5b4e8f6d4b49df337228c
2 files changed