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>

mem: hmc: adds controller

This patch models a simple HMC Controller. It simply schedules the incoming
packets to HMC Serial Links using a round robin mechanism. This patch should
be applied in series with other patches modeling a complete HMC device.

Committed by: Nilay Vaish <nilay@cs.wisc.edu>