Support multiple OPAL consoles, and don't crash if uart is not stdout

[jhibbits]

Summary

If the chosen console is not the OPAL uart, but OPAL uart devices exist, the console device doesn't attach properly, and faults in the interrupt handler, with a NULL pointer dereference. To fix this, and as a byproduct, also support multiple OPAL consoles, refactor to have the console getc callback use the appropriate softc instead of the global console_sc, which may be NULL in the case of a different device being the console.

[nwhitehorn]

Assuming you have tested this and it works, looks great.