build(psci): add build option for OS-initiated mode

Change-Id: Ie4f7b6a36926ab075ebb9c6507a3ff48ce5538fe
Signed-off-by: Wing Li <wingers@google.com>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index d8c6210..1923d19 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -258,6 +258,9 @@
 # Flag used to choose the power state format: Extended State-ID or Original
 PSCI_EXTENDED_STATE_ID		:= 0
 
+# Enable PSCI OS-initiated mode support
+PSCI_OS_INIT_MODE		:= 0
+
 # Enable RAS support
 RAS_EXTENSION			:= 0