add some documentation of the software side of DMA
add some documentation of the software side of DMA

Change-Id: Icf107ec4f45ba152973b9bf2b7aef201e17c3dfa

csail-csg/connectal,cambridgehackers/connectal,8l/connectal,chenm001/connectal,hanw/connectal,8l/connectal,cambridgehackers/connectal,cambridgehackers/connectal,cambridgehackers/connectal,chenm001/connectal,8l/connectal,cambridgehackers/connectal,chenm001/connectal,hanw/connectal,8l/connectal,hanw/connectal,8l/connectal,csail-csg/connectal,hanw/connectal,csail-csg/connectal,csail-csg/connectal,hanw/connectal,csail-csg/connectal,chenm001/connectal,chenm001/connectal