MAINTAINERS: Add the dedicated maillist info for LKMM

A dedicated mail list has been created for Linux kernel memory model
discussion, which could help people more easily track memory model
related discussions.  This could also help bring memory model discussions
to a broader audience.  Therefore, add the list information to the LKMM
maintainers entry.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Andrea Parri <parri.andrea@gmail.com>