add missing s390x files to MAINTAINERS

# gpg: Signature made Tue 21 Oct 2014 11:57:12 BST using RSA key ID C6F02FAF
# gpg: Good signature from "Cornelia Huck <huckc@linux.vnet.ibm.com>"
# gpg:                 aka "Cornelia Huck <cornelia.huck@de.ibm.com>"

* remotes/cohuck/tags/s390x-20141021:
  s390x: sweep up unmaintained files

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>