ARM: dts: imx28-evk: Enable touchscreen support

Provide 'lradc-touchscreen-wires' property to the LRADC driver, so that
touchscreen can be functional.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>