[util/tlgen] Allow user to specify async fifo depth

The asycn fifo depth inside tlgen is hardcoded right now, but it would be good if in the future top levels can specify how deep it should be directly.

Start by locating tlgen and the hardcoded async FIFO depth, then trace how top-level configuration is represented and passed into generated output. Done means a top level can specify the FIFO depth directly and tlgen uses that value instead of the hardcoded depth.

Written by the indexing model from the issue text.