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.com


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