Use ad_iobuf instance on system top, instead of separate IOBUF instances.
Get rid of unwanted whitespaces.
Modified ZED constraints to 250 MHz for the clock from AD9361
ZC706 runs rx_clk at 250 MHz. ZC702 and ZED run rx_clk at 200 MHz due to slower fabric. The ZC702 and ZED projects need init_user in the boot procedure in order for the HP Ports to work correctly. Both DDS and DMA mode work.