After upgrading to the 4.19.104-mvebu kernel I also started experiencing the same complete cpu lockups, but here's the kicker: the lockups are continuing, even after downgrading to kernel 4.14.171-mvebu.
Easy way for me to trigger it is by running echo check > /sys/block/md0/md/sync_action
I managed to catch the CPU stall via the serial console: https://pastebin.com/C8yCQMAn
Here's the output from armbianmonitor -u:
same lockup still happens when running an md data-check even after downgrading further, to 4.14.153-mvebu.