Refactored the location of the atomic classes.

Moved atomic classes into an SMP directory to make sure that
the SMP backends are obvious for what they are. Also added
a Simple backend in the CMake file which currently is the same
as Sequential.

Change-Id: I7aa0262ea37da774acbe9c2ca13c8e5134178e49
Merge topic 'atomic-refactoring' into master

55fab3d5 Refactored the location of the atomic classes.