ASoC: dt-bindings: imx-hdmi: Add binding doc for hdmi machine driver

Imx-hdmi is a new added machine driver for supporting hdmi devices
on i.MX platforms. There is HDMI IP or external HDMI modules connect
with SAI or AUD2HTX interface.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1607251319-5821-1-git-send-email-shengjiu.wang@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>