dt-bindings: display: bridge: add it66121 bindings

Add the ITE bridge HDMI it66121 bindings.

Signed-off-by: Phong LE <ple@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419071223.2673533-2-narmstrong@baylibre.com