Wire up fsi_bit_bang part

The OpenBMC FSI device driver needs to know which BMC GPIOs are
wired to which FSI related functions, like data, clock, mux, etc.
So, an fsi_bit_bang part which has specifically named gpio slave units
will accept the GPIOs from the BMC, and then has an fsi-master unit
that can be wired to the processor's fsi-slave.