ASoC: bindings: fsl-asoc-card: add compatible string for WM8958 codec

The WM8958 codec is used on i.MX7D val board.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1615986303-27959-2-git-send-email-shengjiu.wang@nxp.com


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