psci: platform control of SYSTEM_SUSPEND entry

Some platforms can only resume from system suspend from the boot
CPU, hence they should only enter that state from that same core.

The following commit presents an interface that allows the platform to
reject system suspend entry near its very last stage (last CPU).

lib/psci/psci_main.c

@@ -167,8 +167,14 @@ int psci_system_suspend(uintptr_t entrypoint, u_register_t context_id)
 	/* Query the psci_power_state for system suspend */
 	psci_query_sys_suspend_pwrstate(&state_info);
 
+	/*
+	 * Check if platform allows suspend to Highest power level
+	 * (System level)
+	 */
+	if (psci_find_target_suspend_lvl(&state_info) < PLAT_MAX_PWR_LVL)
+		return PSCI_E_DENIED;
+
 	/* Ensure that the psci_power_state makes sense */
-	assert(psci_find_target_suspend_lvl(&state_info) == PLAT_MAX_PWR_LVL);
 	assert(psci_validate_suspend_req(&state_info, PSTATE_TYPE_POWERDOWN)
 						== PSCI_E_SUCCESS);
 	assert(is_local_state_off(