ARM: dts: sun5i: q8-tablet: Add LCD Panel enable GPIO

Now that we support the AXP209 GPIOs, we can toggle the LCD panel enable
line. Add the GPIO phandle to the panel.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>