arch: support issuing Atomic Mem Operation (AMO) requests

arch: support issuing Atomic Mem Operation (AMO) requests

This patch adds two new functions named amoMemAtomic used in Atomic CPU
model and initiateMemAMO used in Timing CPU models to issue AMO requests
to the memory system.

Change-Id: I962bdc0e5f25b72c170d96ebc136f29a2b230d5e
Reviewed-on: https://gem5-review.googlesource.com/8186
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>