ASoC: sgtl5000: Add 'clocks' entry as a required propery

Since commit 9e13f345 (ASoC: sgtl5000: Let the codec acquire its clock) , the
'clocks' entry is mandatory, so update the binding doc.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/sound/sgtl5000.txt

@@ -5,9 +5,12 @@ Required properties:
 
 - reg : the I2C address of the device
 
+- clocks : the clock provider of SYS_MCLK
+
 Example:
 
 codec: sgtl5000@0a {
 	compatible = "fsl,sgtl5000";
 	reg = <0x0a>;
+	clocks = <&clks 150>;
 };