mem: Perform write merging in the DRAM write queue

mem: Perform write merging in the DRAM write queue

This patch implements basic write merging in the DRAM to avoid
redundant bursts. When a new access is added to the queue it is
compared against the existing entries, and if it is either
intersecting or immediately succeeding/preceeding an existing item it
is merged.

There is currently no attempt made at avoiding iterating over the
existing items in determining whether merging is possible or not.