ARM: dts: imx6dl: prtvt7: Adjust default backlight brightness to 65

This commit changes the default brightness level of the backlight on the
prtvt7 device from 20 to 65, roughly equivalent to 75% of the maximum
brightness level defined in the "brightness-levels" property.

The adjustment provides a more optimal default brightness for the
device, improving the overall user experience.

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>