ARM: dts: imx28-tx28: Pass #sound-dai-cells

Per sgtl5000.yaml, '#sound-dai-cells' is a required property.

Pass it to fix the following dt-schema warning:

imx28-tx28.dtb: sgtl5000@a: '#sound-dai-cells' is a required property
	from schema $id: http://devicetree.org/schemas/sound/sgtl5000.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>