nrf: port: add memory barrier before wfi

ARM recommends issuing a DSB instruction propr to issuing WFI, as it is
required on many parts suchas Cortex-M7. This is effectively a no-op on
the Cortex-M4 used in most NRF parts, however it ensures that we won't
be surprised when new parts come out.

See
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0321a/BIHICBGB.html
for more information.

Signed-off-by: Sean Cross <sean@xobs.io>

Author: xobs

ports/nrf/supervisor/port.c

@@ -273,8 +273,10 @@ void port_sleep_until_interrupt(void) {
         // available, even though the actual handler won't fire until
         // we re-enable interrupts.
         common_hal_mcu_disable_interrupts();
-        if (!tud_task_event_ready())
+        if (!tud_task_event_ready()) {
+            __DSB();
             __WFI();
+        }
         common_hal_mcu_enable_interrupts();
     }
 }