ARM: dts: imx6: RDU2: add eGalax touchscreen

This touchscreen is present instead of the RMI4 touch on some SKUs of
the RDU2. Keep it disabled by default, the bootloader will enable it
instead of the RMI4 touch when running on one of those units.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>