ARM: dts: imx6qdl-vicut1/vicutgo: Add backlight_led node

backlight_led is the dimmable backlight for the rubber border on the case. It
is also used to highlight the power- and some other buttons.

MX6QDL_PAD_SD4_DAT1__PWM3_OUT is also assigned as output for pwm3. Since
we need pwm3 for the backlight, we're forced to disable user space hardware
revision detection. The bootloader will have to supply this information
(i.e. through device tree).

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>