Hi Manu
one can look at i.MX6/7/8 series USB Certification Guide
PCIe Certification Guide for i.MX8 Serials
availabale on https://www.nxp.com/products/processors-and-microcontrollers/arm-processors/i-mx-applications-proces...
other guides unfortunately are not available, but nxp can help with certification process using
https://contact.nxp.com/new-prof-svcs-sw-tech
Best regards
igor