dt-bindings: display: Convert Allwinner DSI to a schema

The Allwinner SoCs have a MIPI-DSI and MIPI-D-PHY controllers supported in
Linux, with a matching Device Tree binding.

Now that we have the DT validation in place, let's convert the device tree
bindings for that controller over to a YAML schemas.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527120910.18964-1-maxime.ripard@bootlin.com