MAINTAINERS: Add Daniel Lustig as an LKMM reviewer

Dan runs the RISC-V memory model working group.  I've been forwarding
him LKMM emails that end up in my inbox, but I'm far from an expert in
this stuff.  He requested to be added as a reviewer, which seems sane to
me as it'll take a human out of the loop.

CC: Daniel Lustig <dlustig@nvidia.com>
Acked-by: Daniel Lustig <dlustig@nvidia.com>
Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>