drm/mcde: Add device tree bindings

This adds the device tree bindings for the ST-Ericsson
Multi Channel Display Engine MCDE as found in the U8500
SoCs.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416142844.12038-1-linus.walleij@linaro.org