MAINTAINERS: sun4i-drm is now maintained in drm-misc

sun4i-drm is maintained in drm-misc as a small driver.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908194039.882-1-maxime.ripard@free-electrons.com