ASoC: dt-bindings: imx-card: Add binding doc for imx sound card

ASoC: dt-bindings: imx-card: Add binding doc for imx sound card

Imx-card is a new added machine driver for supporting
ak4458/ak5558/ak5552/ak4497 codec on i.MX platforms. But these
DAC/ADCs are not only supported codecs. This machine driver is
designed to be a more common machine driver for i.MX platform,
it can support widely cpu dai interface and codec dai interface.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1621247488-21412-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>