ASoC: fsl: fix the binding of imx-sgtl5000

The sgtl5000 data sheet does not particularly mention that the Mic Bias
will internally supplies to MIC_IN, so fix the example in imx-sgtl5000
binding document to have a better reflection of the actual circuit.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>