MAINTAINERS: Update maintainers for new Documentation/litmus-tests

This commit adds Joel Fernandes as official LKMM reviewer.

Acked-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Apply Joe Perches alphabetization feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>