arch: Get rid of the generic mmapped IPR mechanism.

arch: Get rid of the generic mmapped IPR mechanism.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I4ab6f80581eee39e90fb91c672eca8e1a8fd9046
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23186
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>