nios2: fix irq_lock/unlock ordering bug

Memory accesses could be reordered before an irq_lock() or
after an irq_unlock() without the memory barriers.

See commit 15bc5377 for the
ARM fix for a complete description of the issue and fix.

Change-Id: I1d96fe0088d90150f0888c2893d017155fc0a0a7
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
(cherry picked from commit bba445b3)