[ipgen/multitop/dtgen/reggen] Issues related having instances with different parameters

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### Description

Currently if an IPgen IP is instantiated more than once but with different parameters, those will be treated as different IPs by reggen/dtgen. This creates a lot of friction on the software side because we usually want to treat them together. For example two instances of `rv_core_ibex` or `pwm` are sufficiently close that the differences can be covered by the DT.

Issues:
- trying rename the core from `rv_ibex_core` to something else fails because `topgen` has some assumptions on the name
- renaming the PLIC crashes topgen
- more generally topgen/dtgen has many assumptions about names
- for IPgen IPs, topgen [enforces](https://github.com/lowRISC/opentitan/blob/master/util/topgen.py#L2620) that the `name` matches the `type` which prevents using an IP type more than once
- topgen should check that module names are unique, at the moment this check is indirect by a failed assertion in the xbar elaboration code

Start with util/topgen.py around line 2620 and trace how topgen, dtgen, and reggen handle IPgen instances, names, and types. Review the xbar elaboration path mentioned in the issue; done means renamed or differently parameterized instances are handled consistently and duplicate module names receive an explicit check.

Written by the indexing model from the issue text.