dt-bindings: display: Add Allwinner MIPI-DSI bindings

The Allwinner SoCs usually come with a DSI encoder. Add a binding for it.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cdc5402570199f5c08211f29d9182ea5948d3c40.1522835818.git-series.maxime.ripard@bootlin.com