MFC r336837: Be more conservative about setting hw.uart.console

Note when we've found a 8250 PNP node. Only try to set hw.uart.console
if we see one (otherwise ignore serial hints). The 8250 is the only
one known to have I/O ports, so limit the guessing to when we've
positively seen one. And limit this to x86 since that's the only
platform where we have I/O ports. Otherwise, we'd set the serial port
to something crazy for the platform and fall off the cliff early in
boot.