ASoC: bindings: fsl-asoc-card: add compatible string for si476x codec

The si476x codec is used for FM radio function on i.MX6
auto board.
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1606708668-28786-2-git-send-email-shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/fsl-asoc-card.txt

@@ -40,6 +40,8 @@ The compatible list for this generic sound card currently:
 
  "fsl,imx-audio-tlv320aic32x4"
 
+ "fsl,imx-audio-si476x"
+
 Required properties:
 
   - compatible		: Contains one of entries in the compatible list.