configs: Create a basic ObjectList

configs: Create a basic ObjectList

There are multiple files that reimplement the same functionality
of listing and getting available modules for class instantiation.
Create a base class that can be derived and reduce code duplication.

Change-Id: I96bf47b1ffd10893691b0b95591969b38894dd65
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20588
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>