sim: Add a set_parent to NullSimObject which does nothing.

sim: Add a set_parent to NullSimObject which does nothing.

This lets attempts to set_parent on NullSimObject fall away harmlessly
without having to add a bunch of checks whenever set_parent is called.

Change-Id: I6d3510772ba71e765c4739e486e9f3d2460c4e11
Reviewed-on: https://gem5-review.googlesource.com/4840
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>