Hello Jean Valjean,
You can develop hardware using the i.MX6UL without problems even if you do not have access to the i.MX6UL Security Reference Manual. What this document provides is details on encryption and security systems available for the i.MX6UL. If your application does not need these features you wouldn’t require this document.
If you do need these features you would need to sign an NDA and request access to the document. You can sign an NDA with your NXP Distributor.
I hope this information helps.
Regards,