dt-bindings: panel: td028ttec1: add backlight property

This adds an additional backlight property as described
in panel-common.txt

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>