ARM: dts: imx6ul-isiot-common: Add touchscreen node

Add touchscreen node as i2c1 slave device on Engicam Is.IoT MX6UL
modules, the touchscreen controlled 'st,stmpe-ts' connected via
i2c with st,stmpe811 mfd interface.

Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>