Adds support for Digilent Nexys Video FPGA board.

[milovanovic]

Introduces initial support for Digilent's Nexys Video FPGA development board.

Related PRs / Issues:

Nexys Video support recently added to the chipsalliance/rocket-chip-fpga-shells#22

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?

[jerryz123]

I see no problems here, and the docs are excellent.