dt-bindings: media: Document bindings for HDMI RX Controller

Document bindings for the Synopsys DesignWare HDMI RX Controller.

Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shreeya Patel <shreeya.patel@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>