Added 1:1, 1:2 and 2:1 I/O configurations to fbdelay

The fbdelay unit now supports 1:1, 1:2 and 2:1 I/O configurations,
as well as the originally hardwired 2:2. The unit still provides
the same (stereo) control registers, and internal stereo delay
buffers, regardless of configuration.

Closes #134.