My particular uart resports (status1 == 6 && status2 == 6). If that is good 
enough I can add another porttype and add the correct register address as 
UART_EXAR_EFR in the header file.

Try it and see. We can put it in -next for a while and then there will a
release cycle duriing which any unfortunate clashes wil get resolved.

Alternatively if it's a device attached to a specific platform create it
as a platform device of some kind in the board specific code and pick
that up in the serial driver.

Another thought could be to write 0xFF (or some other value) to the correct 
EFR-register and read that byte to test if the register is present. The 
problem here is that I do not how the bus reacts to writting/reading invalid 
addresses.

The trouble is an extra register may in fact be part of some other device
- it's generally not a great idea if there are other choices.

Alan