Comment # 8 on bug 1204063 from Ivan Ivanov

I have found the right combination of parameters and device tree 
content which activates earlycon for VAR-SOM-MX8 without upstream
Linux modifications.

'earlycon' have to be passed to kernel command line via GRUB and NOT
via device tree like this:

          chosen {
                  bootargs = "earlycon";

Then just make sure you have following into chosen device tree node:

          chosen {
                  stdout-path = "serial0";