dt-bindings: display: imx: hdmi: Convert binding to YAML

Convert the i.MX6 HDMI TX text binding to YAML.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>