mem: Update DRAM controller comments
mem: Update DRAM controller comments

Update comments and add a reference for more information.
(transplanted from 4966471a1ba19dc01e2709d13a4d0cb3774aa468)

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5