newer ultras and probably other sun machines. This is a pretty decent
chip with 32 byte read and write fifos and support for many higher baud
rates. It supports hdlc and sdlc as well as asynchronoue serial, but the
goal here is just decent serial console support for the sparc64 port, so
these modes are not expected to be supported. So far this only has support
for the low level routines used by kernel printf and thus ddb over serial,
a character device which can be used for login consoles has not yet been
written.
