[topgen] Topgen fails if all source clocks are free running

### Description

I was trying no generate a new toplevel in which the source clocks would be external and free running, although derived clocks would still be managed.
The topgen however cannot populate the parent_child_clks dict because of lines 321 and 322 of util/topgen/clocks.py :

`if not clk.aon:`
` parent_child_clks[name] = [name];`

I'm not sure if the field 'aon' is being used correctly or if external free-running source clocks are not allowed in the design as-is since there is an overlap in the meaning of the hjson field 'aon' for free-running clocks and the defined low-power 'aon' always-on clock.

Would just like to understand the rationale.
Thanks

Start in util/topgen/clocks.py at lines 321-322 and trace how the parent_child_clks dictionary is populated for source clocks. Reproduce topgen with external, free-running source clocks, then determine the intended relationship between the hjson aon field and free-running clocks; done means the behavior or required restriction is clearly established and the failure is addressed.

Written by the indexing model from the issue text.