MAINTAINERS: Add simpledrm driver co-maintainer

Thomas asked me to serve as co-maintainer for the simpledrm driver.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220505172610.338299-1-javierm@redhat.com