dev: Add a special reset interface to consolidate reset logic

How to reset a model correctly is very different between models. Take
cpu models for instance, they have different reset pins for different
parts(typically one for each core, one for shared component, one for
debug interface). To make users more easily to reset the model, here we
want to introduce a special reset port. By implementing the port, users
can simply request a whole reset to the model. If users want to do
partial resets, users still can access the raw pins to achieve what they
want.

Change-Id: I746121d16441e021dc3392aeae1a6d9fa33d637a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58810
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed