mem-ruby: Fix const copy of addr range in AbstractController

mem-ruby: Fix const copy of addr range in AbstractController

Clang 10 throws the following error:
  loop variable 'addr_range' of type 'const AddrRange' creates a
  copy from type 'const AddrRange' [-Werror,-Wrange-loop-construct]

  note: use reference type 'const AddrRange &' to prevent copying

Issue introduced by c7fabb979c09864ab47ae848381d6797b45a4cc6.

Change-Id: I43e8d613eb5069d5ce9cb12ddec18cba0a3847f6
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38495
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>