The OP-TEE on Raspberry Pi 3 webpage says,
This port of ARM Trusted Firmware and OP-TEE to Raspberry Pi3 IS NOT SECURE!
Although the Raspberry Pi3 processor provides ARM TrustZone exception states, the mechanisms and hardware required to implement secure boot, memory, peripherals or other secure functions are not available. Use of OP-TEE or TrustZone capabilities within this package does not result in a secure implementation.
This package is provided solely for educational purposes.
The description seems a little vague to me. What mechanisms and hardware do they talk about? Why does RPi3 fall short to provide a secure implementation?