Add info on Device Tree files for CM4

[Liz Upton]

https://github.com/raspberrypi/documentation/edit/develop/documentation/asciidoc/computers/compute-module/cm-peri-sw-guide.adoc

Thanks Nate!

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Alasdair Allan]

No so fast github bot.