Interesting. All my boards reboot at 437.5MHz@1.0V without any problems. But as said, it can totally be that this depends on various random factors (different batch of RAM chips maybe, for example) as officially the chip should run 1350MHz@1.0V only.
The best would be to fix this in TF-A and have bl2 reset the registers in charge of cpu frequency and voltage to their defaults before initiating the (proprietary) DDR RAM calibration which hangs otherwise...