MAINTAINERS: add Melissa to V3D maintainers

I've been contributing to v3d through improvements, reviews, testing,
debugging, etc. So, I'm adding myself as a co-maintainer of the V3D
driver.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Acked-by: Juan A. Suarez <jasuarez@igalia.com>
Acked-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220429193317.ol3u4e5jpt5jucox@mail.igalia.com