Control FPGA new pin in N210

Dear all, I am using N210, I wan to bring “AA17” pin of FPGA to up
3.3v, and how can use UHD to do that?
Josh replied me that make a new settings register in u2plus_core.v
and use set_user_register() to control it. but I read the
u2plus_core.v code, and try to add the setting_reg to control “AA17”
pin, but how can I know the “addr” that multi_usrp.cpp can control it?
could someone can give me an example? thanks a lot.Jack.