mm : solved performance issue by changing write back

Change write through to write back due to performance enhancement.
As a result, coremark benchmark score will be the same on andale port.
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native
Iterations/Sec : 3529
Iterations : 60000

linux on hypervisor
Iterations/Sec : 3529
Iterations : 60000
Merge pull request #125 from joohyun0115/v1

mm : solved performance issue by changing write back