There appears to be no documentation for the TRNG peripheral. The MIMXRT1021 Reference Manual describes what it does, and shows how to enable the clocks _to_ the peripheral etc.
But nowhere is there any documentation on the peripheral itself.
Please can you tell me where I can find it?
It may be that NXP requires an NDA for the details, in which case you will need to contact your NXP FAE.
However, the TRNG in the i.MX RT is the same as the one used in various Kinetis parts and so it can essentially be found in documents such as the K82's reference manual, where you will see the peripheral memory layout is identical.
[uTasker project developer for Kinetis and i.MX RT]