Just to answer my own question partly:
/dev/rckncm is device descriptor for ("rocky") non-cache-able memory. I presume this mean the configuration registers (CRBs). I am not sure what else would be on the list.
/dev/rckncm, as you discovered, is Rock Creek non-cacheable memory. It is used to access the configuration registers and shared memory. The RCCE source code gives good examples of how to do this.