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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>