arm64: dts: colibri-imx8x: Add ad7879_ts label to touchscreen controller

The device tree defines the touchscreen controller, but it cannot be
enabled because it lacks a reference label.
This commit adds a label to allow it to be referenced and enabled.

Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>