There are four uarts on radxa, and the position of them like the follow picture . It's the same for radxa rock, radxa rock pro and radxa rock lite.
In default, the uart2 is used as ttyFIQ0 for debug. if you want to use it ,you should unselect the FIQ DEBUG in kernel and remove the ttyFIQ in parameter.
I suggest you don't use this uart.