xhci(4): Draft suspend/resume.

Work almost entirely done and tested by maya@ based on xhci 1.2 spec;
tidied up and tweaked by me.

Not sure about issuing Stop Endpoint commands or ensuring the Command
Ring is in the Stopped or Idle state, but this seems to work as is,
so it's already an improvement over what we had before which was no
xhci suspend/resume at all.

In particular, it's not clear to us:

- if we don't have any pending USB activity whether we need to issue
  the Stop Endpoints or quiesce the command ring; but

- if we do have any pending USB activity whether issuing Stop
  Endpoint is enough or whether we also need to do anything to
  synchronize with other software logic to quiesce it too.

(riastradh)