SPM: Add idle Partition for single core topology

This patch adds an idle Partition. When all other Partitions are
waiting for signals, this Partition will be scheduled and it simply
waits for interrupts.

Note: Multi-core topology has already the idle thread.

Change-Id: I862eaccb33f4f119fc75a75cd25dc0ddc7869554
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/secure_fw/spm/include/spm_partition_defs.h b/secure_fw/spm/include/spm_partition_defs.h
index 07f224e..c811cb6 100644
--- a/secure_fw/spm/include/spm_partition_defs.h
+++ b/secure_fw/spm/include/spm_partition_defs.h
@@ -24,6 +24,7 @@
  * operations.
  */
 #define TFM_SP_NON_SECURE_ID (0)
+#define TFM_SP_IDLE_ID       (1)
 /* A dummy partition for TFM_SP_CORE is created to handle secure partition
  * calls done directly from the core, before NS execution started.
  */