sim: Add a function for decoding the field(s) of an m5op address.

sim: Add a function for decoding the field(s) of an m5op address.

These have at one time included both a func and subfunc, although the
subfunc was unused and is now excluded.

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

Change-Id: Ic35ced7a012aa72af5454768f3cbd11b431b061a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23183
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Marjan Fariborz <mfariborz@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>