Microwatt may have firmware that places data in r3, which was used to
detect microwatt vs powernv. This breaks the existing probing of the UART
type in this powerpc port.
Instead build only the appropriate UART into the firmware, selected by
passing the option UART=potato or UART=lpc_serial to the Makefile.
A future enhancement would be to parse the device tree and configure
MicroPython based on the settings.