You've got that the wrong way around - the PSP is able to access arbitrary areas of physical memory that can only otherwise be accessed via SMM, and so compromising the PSP gives you the ability to tamper with SMM code (as discussed in https://blog.trailofbits.com/2018/03/15/amd-flaws-technical-...). Having access to SMM doesn't give you access to the PSP, and being able to modify flash doesn't circumvent Platform Secure Boot. Power off the board and the CPU will return to exactly the state it left the factory in.