drm/tve200: Add DT bindings

This adds device tree bindings for the Faraday TVE200 IP block.
This IP block is present in the Gemini ARM SoC and also in some
Grain Media GMxxxx SoCs.

Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170820100557.24991-1-linus.walleij@linaro.org