Notch out known-unsupported PMBus status registers

[jamesmunns]

This PR adds a couple of "Illegal" register definitions for known-absent registers.

oxidecomputer/hubris#2538 begins looking for this information when deciding whether to query this field or not, there will likely be some follow-on PRs to add for more devices.

[evan-oxide]

Hello I pushed an extra commit for the MWOCP67. I tested that with this branch and oxidecomputer/hubris#2549, get_pmbus_status works on the Observer and doesn't set a STATUS_CML bit (which it would if it had tried to read STATUS_FANS_3_4).

[hawkw]

@evan-oxide I think your commit looks great. We should really go ahead and merge this --- in James' absence, would you like to do the honors?

[jamesmunns]

Ah shoot, I thought this had gotten merged as part of my release train for the pmbus status work. The fact that this didn't means that the version of get-pmbus-status that is merged in Hubris right now will still try to query the "illegal" registers for these devices.

Someone will need to do a follow-up PR for hubris that does a cargo update -p pmbus to make sure we have this PR's work included before the R21 release.

[evan-oxide]

I'm bumping it as part of hubris#2549 observer: MWOCP67 PMBus Support, which will hopefully be merged soon.