Add EmptyChipTop example

[jerryz123]

This shows how to replace the soc-oriented ChipTop with an empty one that can be filled with whatever the user wants.

Useful for teaching people how the system works, or for setting up one-off experiments.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[joonho3020]

LGTM

[abejgonzalez]

SFTM.

[abejgonzalez]

Then why keep this then?

[jerryz123]

Its kind of ugly... you can't do

extends Config(
  new chipyard.example.WithEmptyChipTop
)

By itself. You'd have to do

extends chipyard.example.withEmptyChipTop

But this is confusing to users. I elected to follow the pattern