css: Ensure PSCI system off/reset is not interrupted

If there is a pending interrupt, it is possible for the AP to come out
of the final WFI before SCP has a chance to act on it.  Prevent this
by disabling the GIC CPU interface before issuing a WFI.

Previously, SCP would not wait on WFI before taking an action but
would shut down the core or system regardless.

Change-Id: Ib0bcf69a515d540ed4f73c11e40ec7c863e39c92
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

plat/arm/css/drivers/scp/css_pm_scpi.c

@@ -32,6 +32,7 @@
 #include <assert.h>
 #include <css_pm.h>
 #include <debug.h>
+#include <plat_arm.h>
 #include "../scpi/css_scpi.h"
 #include "css_scp.h"
 
@@ -134,6 +135,12 @@ void __dead2 css_scp_sys_shutdown(void)
 {
 	uint32_t response;
 
+	/*
+	 * Disable GIC CPU interface to prevent pending interrupt
+	 * from waking up the AP from WFI.
+	 */
+	plat_arm_gic_cpuif_disable();
+
 	/* Send the power down request to the SCP */
 	response = scpi_sys_power_state(scpi_system_shutdown);
 
@@ -153,6 +160,12 @@ void __dead2 css_scp_sys_reboot(void)
 {
 	uint32_t response;
 
+	/*
+	 * Disable GIC CPU interface to prevent pending interrupt
+	 * from waking up the AP from WFI.
+	 */
+	plat_arm_gic_cpuif_disable();
+
 	/* Send the system reset request to the SCP */
 	response = scpi_sys_power_state(scpi_system_reboot);