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>

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