Documentation/litmus-tests: Introduce atomic directory

Although we have atomic_t.txt and its friends to describe the semantics
of atomic APIs and lib/atomic64_test.c for build testing and testing in
UP mode, the tests for our atomic APIs in real SMP mode are still
missing. Since now we have the LKMM tool in kernel and litmus tests can
be used to generate kernel modules for testing purpose with "klitmus" (a
tool from the LKMM toolset), it makes sense to put a few typical litmus
tests into kernel so that

1)	they are the examples to describe the conceptual mode of the
	semantics of atomic APIs, and

2)	they can be used to generate kernel test modules for anyone
	who is interested to test the atomic APIs implementation (in
	most cases, is the one who implements the APIs for a new arch)

Therefore, introduce the atomic directory for this purpose. The
directory is maintained by the LKMM group to make sure the litmus tests
are always aligned with our memory model.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>