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With the current implementation, it's possible for a contender to
observe accesses in the Critical Section before acquiring or releasing
the lock. Insert fencing in the locking and release codes to prevent any
reorder.

Fixes ARM-software/tf-issues#609

Change-Id: I773b82aa41dd544a2d3dbacb9a4b42c9eb767bbb
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>