add more buffer sizes for dram test

- depending on L1/L2 cache size, each case will show different results.
- For current devices, 16MB (= total 32MB for src + dst) should be big
  enough to exceed L2 cache size.
- Sudden drop in performance, when starting from 4KB will tell that a cache
  size has passed.

Change-Id: I5fa47adb92bb3d62a4af39b6ba5dcc478fba340a