[coreboot] Servo debug uart in Linux?

[Trammell Hudson]

Is it possible to use the Skylake Servo debug UART in Linux or Xen?
It doesn't show up as a normal 16550 (setserial reports "uart type
unknown"), which is making debugging the payload kernel a little
frustrating.

I've added lots of "outb $0x80" calls to trace the Xen hypervisor and
have figured out that the Skylake doesn't work with the x2apic, the xsave
instruction or the MCE handler, but would prefer a higher bandwidth way
to report progress/errors.