Unlike the other Sun machines, UltraSPARCs can have consoles run on different
chips than zs, so we need to support them. So, here we go:
Add a new PROM console driver with a major number and everything.
This is the default driver if nothing else attaches. It does not
use the keyboard driver since the PROM translates keystrokes itself.
(Unfortunately it also swallows L1-A).
Have the keyboard driver take over the console when it attaches on a
serial port. When a serial port detects a keyboard and attaches the
keyboard driver, it needs to provide a set of consdev vectors. They
keyboard driver will use those to send I/O to the keyboard and mouse.
