MAINTAINERS: Add Boris Brezillon as Panfrost driver maintainer

Add Boris Brezillon as Panfrost driver maintainer. Boris is a new lead
developer of the Panfrost Mesa driver and main developer behind the
upcoming Panthor kernel driver that will serve next-gen Mali GPUs.

Remove Tomeu and Alyssa, who left Collabora and stepped down from working
on Panfrost.

Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Tomeu Vizoso <tomeu.vizoso@tomeuvizoso.net>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Acked-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230822052555.538110-1-dmitry.osipenko@collabora.com