Why does opb_arbiter show up as a standalone IP? You automatically get an arbiter when you have an OPB bus instance, do you not?
In EDK 8.1i, OPB Arbiter (opb_arbiter_v1_02_e, opb_arbiter_v1_02_d, opb_arbiter_v1_02_c) is no longer available as a standalone core. It is a library component to the OPB bus core. The OPB bus instantiation automatically includes the latest OPB Arbiter in the design.
If you need this core for future reference, you can copy the IP core from the EDK directory into the "pcore'"directory of your project file.