MAINTAINERS: Add myself as reviewer for mgag200 and ast drivers

I've contributed to these two drivers, fixing bugs and performance
issues.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230530142615.57014-1-jfalempe@redhat.com