The only way to update u-boot I can recommend is to install the latest Android firmware from manufacturer. I haven't tried and I don't recommend flashing u-boot from a different device.S905 devices without freezes are WeTek Hub, WeTek Play2 and Odroid-C2. All my non-branded S905 devices "died" in one or another way, I can't check.
Also S905X devices are affected?
My V88 Pro with S905x comes with android 6 firmware (S905 usualy have 5.1) and is working fine. I don't know if the issue is related with the android version.