Merging plan for SBIv3-dependent patches

[Palmer Dabbelt]

We've talked about this a few times in the patchwork meeting, but I 
figured I'd write up an email as this is a bit of a wacky process.  

There's a bunch of patch sets blocked on the SBIv3 freeze.  That's been 
coming soon for a while now, but it seems like now it's actually really 
coming soon.  So I'm going to start staging those onto a "sbiv3" branch, 
as there's some cross-set dependencies and probably some merge conflicts 
that will need to get sorted out.

I'm going to plan on doing a staging merge of that branch onto the tip  
of for-next, so it can get some time on the testers.  That'll be a 
little different than how we usually handle for-next, but I think it's 
the best way to go as it'll make dropping the patches easy in case the 
specs don't get frozen in time.