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
diff --git a/services/std_svc/spm/el3_spmc/spmc.h b/services/std_svc/spm/el3_spmc/spmc.h
index e891516..df0aa61 100644
--- a/services/std_svc/spm/el3_spmc/spmc.h
+++ b/services/std_svc/spm/el3_spmc/spmc.h
@@ -23,6 +23,8 @@
 #define FFA_SWD_ID_LIMIT	SPMD_DIRECT_MSG_ENDPOINT_ID - 1
 #define FFA_SWD_ID_MASK		0x8000
 
+/* ID 0 is reserved for the normal world entity, (Hypervisor or OS Kernel). */
+#define FFA_NWD_ID		U(0)
 /* First ID is reserved for the SPMC */
 #define FFA_SPMC_ID		U(FFA_SWD_ID_BASE)
 /* SP IDs are allocated after the SPMC ID */