arm: Support the safe WFE sequence for Krait CPUs

Certain version of the Krait processor require a specific
code sequence to be executed prior to executing a WFE
instruction to permit that instruction to place the
processor into a low-power state.

Change-Id: I308adc691f110a323cbd84e9779675ac045826fa
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>