drm/stm: Add STM32 LTDC driver

drm/stm: Add STM32 LTDC driver

This controller provides output signals to interface directly a variety
of LCD and TFT panels. These output signals are: RGB signals
(up to 24bpp), vertical & horizontal synchronisations, data enable and
the pixel clock.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1492164819-10513-5-git-send-email-yannick.fertre@st.com
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Eric Anholt <eric@anholt.net>