mem: Add comments for the DRAM address decoding
mem: Add comments for the DRAM address decoding

This patch adds more verbose comments to explain the two different
address mapping schemes of the DRAM controller.

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin