[topgen] indication of subsystem

Revise `top_{name}.hjson` to have indications of subsystems. Each IP instances, memories, crossbars will have `subsystem` field that indicates which subsystem it belongs. Top-level also has `subsystems` field which has details of the subsystem configs.

With these fields, topgen will be able to figure out what signal ports the subsystem has and create the subsystem RTL accordingly.

Related issues: #295 #892

Start with the topgen handling of `top_{name}.hjson` and review related issues #295 and #892 for the intended subsystem model. Define the instance, memory, crossbar, and top-level `subsystems` fields, then verify that topgen can derive subsystem signal ports and generate the corresponding subsystem RTL.

Written by the indexing model from the issue text.