serial_bcm283x_mu: Fail loading if not muxed

The bcm283x mini-uart is only really usable as U-Boot serial output
when it is muxed to the UART pins of the RPi pin header.

So fail probing in case it is not muxed correctly, as in that case
firmware did not initialize it properly either.

Signed-off-by: Alexander Graf <agraf@suse.de>