That is not what the term "MMU" is usually taken to mean.

The device that controls the DRAM refresh etc is usually called a "DRAM Controller"

A "Memory Management Unit" (MMU) is a higher-level function that gives the memory protection functions required by operating systems such as Windows and UNIX/Linux: These are the functions that allow each Task to run in its own "protected" memory space, and prevent one task from trashing another's memory.
See http://www.8052.com/forum/read.phtml?id=70848