mem-ruby: Add functionalReadBuffers to AbstractController

mem-ruby: Add functionalReadBuffers to AbstractController

Forwards a functional read accesses to all message buffers, similar to
functionalWriteBuffers.

Change-Id: I54b0ba16aab84575e4c9d6102f6c519b309aa95b
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22020
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bradford Beckmann <brad.beckmann@amd.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>