dt-bindings: display: stm32: Add DSI controller

This patch adds documentation of device tree bindings for the STM32 DSI
controller based on the Synopsys DesignWare MIPI DSI host controller.

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500277223-29553-7-git-send-email-philippe.cornu@st.com