sdb for wb_crossbar outside the WRPC
In the reference SPEC design with the WR PTP Core
(/top/spec_1_1/wr_core_demo in the repository) WB crossbar that
multiplexes Gennum and Etherbone with WRPC Slave interface does not use
SDB. In this situation, when loading Linux drivers, they complain about
missing SDB:
spec 0000:01:00.0: FPGA programming successful
fmc_trivial FmcDio5cha-0100: Can't find SDB at address 0x0
It is not critical, because for this simple design the driver does not do more than writing a bitstream to the FPGA. The functionality of WRPC is not affected even if this error is reported.