systemc: Add a stub implementation for sc_attr related classes.

Change-Id: I59ba11f71b5412643ea7026df91587fafa0c6fda
Reviewed-on: https://gem5-review.googlesource.com/10832
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>
3 files changed