Merge remote-tracking branch 'remotes/cohuck/tags/s390x-20200605' into staging

s390x update:
- enhance s390x documentation
- allow ORBs without prefetch specified for vfio-ccw
- various cleanups and enhancements

# gpg: Signature made Fri 05 Jun 2020 16:32:43 BST
# gpg:                using RSA key C3D0D66DC3624FF6A8C018CEDECF6B93C6F02FAF
# gpg:                issuer "[email protected]"
# gpg: Good signature from "Cornelia Huck <[email protected]>" [marginal]
# gpg:                 aka "Cornelia Huck <[email protected]>" [full]
# gpg:                 aka "Cornelia Huck <[email protected]>" [full]
# gpg:                 aka "Cornelia Huck <[email protected]>" [marginal]
# gpg:                 aka "Cornelia Huck <[email protected]>" [marginal]
# Primary key fingerprint: C3D0 D66D C362 4FF6 A8C0  18CE DECF 6B93 C6F0 2FAF

* remotes/cohuck/tags/s390x-20200605:
  target/s390x: Restrict system-mode declarations
  target/s390x/helper: Clean ifdef'ry
  target/s390x: Only compile decode_basedisp() on system-mode
  MAINTAINERS: add Thomas as additional s390x maintainer
  docs/s390x: document vfio-ccw
  vfio-ccw: allow non-prefetch ORBs
  docs/s390x: document 3270
  docs/s390x: document the virtual css
  s390x: pv: Fix KVM_PV_PREP_RESET command wrapper name
  s390x/kvm: help valgrind in several places

Signed-off-by: Peter Maydell <[email protected]>