Improve GCD example

[jerryz123]

With recent rocket-chip bumps. the example GCD MMIO peripheral had to update. Given that many people base their own MMIO peripheral implementations off the example GCD, I thought it best to update the example GCD to match a more "standard" style of peripheral device construction.

This change

• Reduces the number of traits in the example
• Switches to a more straightforward LazyModule + node structure, instead of the opaque (and to-be-deprecated) TLRegisterRouter API
• Sacrifices concision for clarity, the examples for AXI and TL duplicate some code, but this makes each example clearer
• Demonstrates how to declare a peripheral as a ClockSinkDomain, which enables it to be placed under BaseSubsystem without being wrapped in another layer of module hierarchy.

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?

[abejgonzalez]

LGTM. Doc change?

[gonsolo]

https://chipyard.readthedocs.io/en/latest/Customization/MMIO-Peripherals.html still refers to TLRegisterRouter.
I guess that has to be updated too?