Hello ShinjiOhishi,
The Platform Security Architecture is part of the ARM architecture, so we don’t have many documents pertaining it. The persistent keys are storage in isolated or shielded locations so these would not be lost on power off.
For the LPC55SXX in particular, you may see where the keys are stored in the protected flash region in the worksheet attached to its user manual (UM11126).
I hope that this information helps!
Regards,
Gustavo