MAINTAINERS: Add Boqun to RCU entry

Just to be clear, the "M:" tag before my name is short of "Minions" ;-)

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>