mem: Standardize mem folder header guards

mem: Standardize mem folder header guards

Standardize all header guards in the mem directory according to the most
frequent patterns. In general they have the form:
  mem:  __FOLDER_TREE_FILE_NAME_HH__
  ruby: __FOLDER_TREE_FILENAME_HH__

Change-Id: I983853e292deb302becf151bf0e970057dc24774
Reviewed-on: https://gem5-review.googlesource.com/7881
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>