ARM: dts: at91: nattis: initialize the BLON pin as output-low early

The pwm-backlight driver initializes BLON (the enable gpio) to
output-high if the gpio is input on probe. Initializing the gpio
to output-low before the driver probes prevents this action by
the pwm-backlight driver and gets rid of a nasty blink of full
backlight with an uninitialized panel.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>