If I want to add a new platform support in TF-A for RK3566 as an example, what Documentation do I need to read.

Using RK3399 as a contrast ( because most of RK3399 doc is opened in internet ), we already know this SoC is supported in OPTEE and TF-A. And I can get  RK3399 Docs: TRM V1.3 Part 1, TRM V1.3 Part2, TRM V1.4 Part 1, Datasheet V2.1. I can see in TRM chapter 16 System Security, there are some descriptions about system security, and references to other system registers, like SGRF, etc, but it still seems to me insufficiently ( No SGRF description ) to finish a full support platform implementation in TF-A. Some people said I need to sign an NDA with Rockchip to get Security related part docs. But when I reach to Rockchip, they said all docs are opened already, No NDA options.  When I talked to one partner/distributor of Rockchip, only security related doc is also some doc I can find on internet.

So I am curious and confused, can I, as a third party developer, develop a new platform implementation for TF-A / OPTEE ( specially for Rockchip Platform )?

Thanks