hi,
I suppose that you asked the LPC55S69, it is based on Cortex-M33 and has trust-zone module.
yes, there is register to lock/unlcok writing the MPU registers.
Pls refer to section 47.4.75 Miscellaneous control signals for in Primary CPU0 in the UM11126.pdf

Hope it can help you
BR
XiangJun Rong