MFC 271482: Add support for adding empty partition entries.

Relnotes:	yes
Approved by:	re@ (gjb)
MFC r271679:

  Merge atse(4) interrupt handling and race condition fixes from
  cheribsd.

  Obtained from:	cheribsd
  Submitted by:		rwatson, emaste
Sponsored by:		DARPA/AFRL
Approved by:		re (delphij)