Be more conservative about setting hw.uart.console

[imp]

Summary

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.

also added matt macy because he's doing the EPOC stuff which will have a x86 use-case.

[imp]

If someone sets hw.uart.console, assume they know what they are doing
and don't second guess them.

[markj]

This fixes my boot-time hang on ThunderX. I don't know the code well enough to mark it reviewed, but it seems sane to me.