drm/gma500: Remove Medfield support

Medfield is an outdated mobile platform with apparently no users left.
Remove it from gma500.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210129095604.32423-2-tzimmermann@suse.de