parisc: Fix system shutdown halt

On those parisc machines which don't provide a software power off
function, the system currently kills the init process at the end of a
shutdown and unexpectedly restarts insteads of halting.
Fix it by adding a loop which will not return.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # 4.9+

arch/parisc/kernel/process.c

@@ -142,6 +142,8 @@ void machine_power_off(void)
 
 	printk(KERN_EMERG "System shut down completed.\n"
 	       "Please power this system off now.");
+
+	for (;;);
 }
 
 void (*pm_power_off)(void) = machine_power_off;