feat(psci): update PSCI_FEATURES

This patch updates the PSCI_FEATURES handler to indicate support for
OS-initiated mode per section 5.15.2 of the PSCI spec (DEN0022D.b) based
on the value of `FF_SUPPORTS_OS_INIT_MODE`, which is conditionally
enabled by the `PSCI_OS_INIT_MODE` build option.

Change-Id: I5da8a989b53419ad2ab55b73ddeee6e882c25554
Signed-off-by: Wing Li <wingers@google.com>
diff --git a/include/lib/psci/psci.h b/include/lib/psci/psci.h
index d4f3670..6d27b7b 100644
--- a/include/lib/psci/psci.h
+++ b/include/lib/psci/psci.h
@@ -143,7 +143,11 @@
 
 /* Features flags for CPU SUSPEND OS Initiated mode support. Bits [0:0] */
 #define FF_MODE_SUPPORT_SHIFT		U(0)
+#if PSCI_OS_INIT_MODE
 #define FF_SUPPORTS_OS_INIT_MODE	U(1)
+#else
+#define FF_SUPPORTS_OS_INIT_MODE	U(0)
+#endif
 
 /*******************************************************************************
  * PSCI version