The Xbox does not use AMD's Platform Security Processor, they have their own custom controller on die with Microsoft hardware and their own custom BootROM. This custom security processor actually has its own bit of RAM entirely in the die and doesn't even go out to the DRAM at all.
See the slide on Tony Chen's presentation about Xbox One security https://youtu.be/U7VwtOrwceo?t=843 and this video from the person who hacked the Xbox One https://youtu.be/FTFn4UZsA5U?t=299