[arm] Implements platform_halt_cpu for generic ARM

Change-Id: I922336ce94437e47fe333c7ad15241e6949b105c

kernel/platform/generic-arm/platform.cpp

@@ -380,7 +380,7 @@ void platform_halt_cpu(void) {
 #else
 
 void platform_halt_cpu(void) {
-    PANIC_UNIMPLEMENTED;
+    psci_cpu_off();
 }
 
 #endif