MAINTAINERS: add drm/sti driver into drm-misc

drm/sti driver is now part of drm-misc as a small driver.

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491835958-30229-1-git-send-email-vincent.abriou@st.com