arm: turn off cpu on panic

Since interrupts are disabled on panic, linux cannot reset a
panic'd core when NMI are disabled (for e.g. mcreboot/mcstop)

Just always offline it, so linux can get it back

Change-Id: If8107172375f2924e02bd4c36e24645ec38a8999

arch/arm64/kernel/cpu.c

@@ -1274,7 +1274,7 @@ void ihk_mc_delay_us(int us)
 	arch_delay(us);
 }
 
-void arch_print_stack()
+void arch_print_stack(void)
 {
 }
 
@@ -1310,6 +1310,11 @@ void arch_show_interrupt_context(const void *reg)
 	kprintf("  syscallno : %016lx\n", regs->syscallno);
 }
 
+void arch_cpu_stop(void)
+{
+	psci_cpu_off();
+}
+
 /*@
   @ behavior fs_base:
   @   assumes type == IHK_ASR_X86_FS;