ASoC: dt: sgtl5000.txt: Add description for 'reg' field

Add description for 'reg' field.
Signed-off-by: NFabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

Documentation/devicetree/bindings/sound/sgtl5000.txt

@@ -3,6 +3,8 @@
 Required properties:
 - compatible : "fsl,sgtl5000".
 
+- reg : the I2C address of the device
+
 Example:
 
 codec: sgtl5000@0a {