ARM: dts: sun4i: Add backlight node to pov protab2 ips9 tablet

Add a node describing the lcd panel backlight on the pov protab2 ips9
tablet.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>