mem-ruby: GCN3 and VIPER integration

mem-ruby: GCN3 and VIPER integration

This patch modifies the Coalescer and VIPER protocol to support memory
synchronization requests and write-completion responses that are
required by upcoming GCN3 implementation.

VIPER protocol is simplified to be a solely write-through protocol.

Change-Id: Iccfa3d749a0301172a1cc567c59609bb548dace6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29913
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Bradford Beckmann <brad.beckmann@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Bradford Beckmann <brad.beckmann@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>