Cactus: Add third partion to ffa tests

Add the third partition to the ffa_partition_info_get test to test
that a partition can successfully get information about the third cactus
partition

Signed-off-by: Ruari Phipps <ruari.phipps@arm.com>
Change-Id: Ib8c69f16e217f9631be005c7759753d05704afd2
diff --git a/spm/common/sp_helpers.h b/spm/common/sp_helpers.h
index 2b9cc2e..7096daf 100644
--- a/spm/common/sp_helpers.h
+++ b/spm/common/sp_helpers.h
@@ -12,6 +12,8 @@
 #include <ffa_helpers.h>
 
 #define SPM_VM_ID_FIRST                 (1)
+#define SPM_VM_ID_SECOND                (2)
+#define SPM_VM_ID_THIRD                 (3)
 
 #define SPM_VM_GET_COUNT                (0xFF01)
 #define SPM_VCPU_GET_COUNT              (0xFF02)