arm: Put the CPU into a sleep state in the idle loop

The ARM doesn't actually say that the WFI instruction will return
immediately if CPSR.I is clear and an instruction is pending, but
this seems to match up with how linux uses it.

Signed-off-by: Tim Deegan <tim@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

xen/arch/arm/domain.c

@@ -34,7 +34,11 @@ void idle_loop(void)
         if ( cpu_is_offline(smp_processor_id()) )
             stop_cpu();
 
-        /* TODO: (*pm_idle)(); */
+        local_irq_disable();
+        if ( cpu_is_haltable(smp_processor_id()) )
+            asm volatile ("dsb; wfi");
+        local_irq_enable();
+
         do_tasklet();
         do_softirq();
     }