dt-bindings: display: Add STM32 LTDC driver

This patch adds documentation of device tree bindings for the STM32 LTDC
(Lcd-Tft Display Controller).

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1492164819-10513-4-git-send-email-yannick.fertre@st.com