ARM: dts: imx6qdl-sabresd: Add touchscreen support

mx6 sabresd boards have a egalax touchscreen controller connected via I2C3.

Add support for it.

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