Clarify W1. (#372)

* Clarify W1.

The definition used to be vague. I think this is the one that makes the
most sense, and shouldn't cause any problems. (Possibly we don't want
this for ackhavereset. I sent e-mail about that.)

The bits that are defined as W1 are:
DM: ackhavreset, setresethaltreq, clrresethaltreq
JTAG: dmihardreset, dmireset

* Revert my "clarification" of W1.