added a few comments to ZynqTop
added a few comments to ZynqTop

Change-Id: Ice3d1c3f4bbc69d386078c1c8197d0e9c8296c24

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