This is a known problem if you have modified the CPU frequency governor to ondemand or otherwise set the CPU supply voltage to 0.9V which causes DDR RAM calibration in ARM Trusted Firmware bl2 to never complete and hang for ever, which is why the device doesn't come up when you reboot.
If you have not changed the CPU frequency governor or otherwise made sure it will not perform reboot while operating at the lowest operating point, reboot should work and what you describe would be new to me.