NVMe API update

[Chuck Tuffli]

I've put NVMe changes to reconcile the current API difference up for
review here:

https://reviews.freebsd.org/D16404

TL;DR is 11.x is using the original NVMe API while 12.x was changed to
support PPC. This patch makes the 12.x API compatible with 11.x and
works with big-endian. Comments and testing feedback would be most
appreciated as I'd like to get this into the tree before the code
slush begins. TIA!

--chuck