dt: bindings: lp8860: Add trigger binding to the lp8860

Add a default trigger optional node to the child node.
This will allow the driver to set the trigger for a backlight.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>