edited xps_library/\@xps_dram/gen_mhs_ip.m to change the ddr3_controller variable for ROACH2 DDR3_APP to ddr3_ctrl instead of ddr3_app.  this change allows us to insert the OPB interface when previously it was not accounted for.  As of now, no plans to relabel DDR3_APP variable in the ddr3_controller files, but the naming discrepancy is noted.
added opb_dram_sniffer instantiation for ROACH2 into @xps_dram/gen_mhs_ip.m

This commit adds the opb_dram_sniffer for the ROACH2 case,
previously omitted in the gen_mhs_ip.m file.  This alteration,
along with the previously committed port change to the ddr3_contr-
oller instantiation in the gen_mhs_ip.m file fully hooks the
current opb_dram_sniffer under test into the design.