Update doc for GCD changes

[gonsolo]

Background Work

• Yes, I searched the mailing list
• Yes, I searched prior issues
• Yes, I searched the documentation

Chipyard Version and Hash

Release: 1.5.0
Hash: a6a6a6

OS Setup

Ubuntu

Other Setup

Ex: Prior steps taken / Documentation Followed / etc...

Current Behavior

See #1757
The GCD example was updated to not use TLRegisterRouter but the documentation at
https://chipyard.readthedocs.io/en/latest/Customization/MMIO-Peripherals.html
still refers to it.

Expected Behavior

Documentation up to date and in line with implementation.

Other Information

No response

[jerryz123]

Thanks, I missed this when updating. I'll get to this soon

[jerryz123]

Actually if you switch to the main branch, you'll see the new docs.

[gonsolo]

The top of the page still refers to a TLRegisterRouter:

The easiest way to create a MMIO peripheral is to use the TLRegisterRouter or AXI4RegisterRouter widgets, which abstracts away the details of handling the interconnect protocols and provides a convenient interface for specifying memory-mapped registers.

The GCD changes made me think TLRegisterRouter is obsolete.