PSCI: add core_power_down_wfi() hook for late power-off

Without a management processor to pull a core's "plug" remotely, we need
to turn our own power off at some point. Doing this in the middle of
the PSCI handling code during the platform-specific CPU_OFF callback is
a bad idea, as some locks are held at this point. As the core stops executing
instructions immediately when power gated, we never get a chance to
release those locks.
In the style of mainline ATF, introduce a callback function to do some
late platform-specific power-off work. This callback is called as the
last action in the process, just before sending the core to WFI.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

include/bl31/services/psci.h

@@ -163,6 +163,7 @@ typedef struct plat_pm_ops {
 	int (*affinst_suspend_finish)(unsigned long,
 				      unsigned int,
 				      unsigned int);
+	void (*core_power_down_wfi)(unsigned long);
 	void (*system_off)(void) __dead2;
 	void (*system_reset)(void) __dead2;
 } plat_pm_ops_t;

services/std_svc/psci/psci_main.c

@@ -115,8 +115,22 @@ int psci_cpu_suspend(unsigned int power_state,
 				 power_state,
 				 MPIDR_AFFLVL0,
 				 target_afflvl);
-	if (rc == PSCI_E_SUCCESS)
+
+	if (rc == PSCI_E_SUCCESS) {
+		/*
+		 * Perform any platform specific, late power-down actions.
+		 * In case this ever returns (it does not need to), fall
+		 * through to the normal WFI sequence.
+		 */
+		if (psci_plat_pm_ops->core_power_down_wfi) {
+			unsigned long mpidr = read_mpidr_el1();
+
+			psci_plat_pm_ops->core_power_down_wfi(mpidr);
+		}
+
 		psci_power_down_wfi();
+	}
+
 	assert(rc == PSCI_E_INVALID_PARAMS);
 	return rc;
 }
@@ -139,8 +153,20 @@ int psci_cpu_off(void)
 	 * successfully completed. Enter a wfi loop which will allow the
 	 * power controller to physically power down this cpu.
 	 */
-	if (rc == PSCI_E_SUCCESS)
+	if (rc == PSCI_E_SUCCESS) {
+		/*
+		 * Perform any platform specific, late power-down actions.
+		 * In case this ever returns (it does not need to), fall
+		 * through to the normal WFI sequence.
+		 */
+		if (psci_plat_pm_ops->core_power_down_wfi) {
+			unsigned long mpidr = read_mpidr_el1();
+
+			psci_plat_pm_ops->core_power_down_wfi(mpidr);
+		}
+
 		psci_power_down_wfi();
+	}
 
 	/*
 	 * The only error cpu_off can return is E_DENIED. So check if that's