MAINTAINERS: Add myself as a maintainer for vc4

Eric isn't working on vc4 anymore and I've been working on it, as well as
merging patches for it, recently so let's make it official so I don't miss
patches.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20201009074952.11345-1-maxime@cerno.tech