ARM: dts: imx6dl-prtvt7: Remove backlight enable gpio

The backlight power is controlled through the reg_bl_12v0 regulator.

Co-Developed-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>