drm/stm: Add STM32 DSI controller driver

Add the STM32 DSI controller driver that uses the Synopsys DesignWare
MIPI DSI host controller bridge.

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500277223-29553-8-git-send-email-philippe.cornu@st.com