All register writes to the protection unit must be done by a secure bus transaction as defined by TrustZone.
Once the XMPU LOCK [RegWrDis] register bit is set,
access to the XMPU registers is disabled and can no longer be written to until after
a POR reset. The only exception is a secure transaction can write to the interrupt
status register (ISR).
Note: Regardless of the LOCK [RegWrDis] setting, the status registers are
always writable by secure and non-secure transactions. All XMPU registers are
only writable by a secure transaction. The registers are readable by secure or
non-secure transactions.