AXI4 Width Widget development

**Type of issue**: other enhancement

**Impact**: no functional change

**Development Phase**: proposal

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

**What is the current behavior?** Have a draft code

**What is the expected behavior?** Full equipped development

**Please tell us about your environment:** AMBA V-IP Packages

**What is the use case for changing the behavior?** Able to do width convert on AXI4 Bus

If you have other ways to do bus width convert on AXI4 w/o AXI4 Width Widget, let me know.

Otherwise, we need to develop.

No files, tests, or entry points are identified. Start by locating the existing AXI4-related draft code in the Rocket Chip and AMBA V-IP sources, then determine the design needed for AXI4 bus width conversion; done would mean a fully equipped AXI4 Width Widget implementation.

Written by the indexing model from the issue text.