MAINTAINERS: Add our new mailing-list

We've been struggling to get an LF-hosted mailing list for a while, but
now that lists.linux.dev is there we opted in.

Let's add it to MAINTAINERS.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://lore.kernel.org/r/20210331130830.64182-1-maxime@cerno.tech