ARM: dts: sun5i: Add backlight node to sun5i-q8-common.dtsi

All A13 based q8 formfactor tablets use the same backlight setup, add
a backlight devicetree node for controlling the backlight on these devices.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>