ARM: dts: sunxi: Add missing backlight supply

The pwm-backlight binding requires a power supply. Make sure we provide
one.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://lore.kernel.org/r/20210114113538.1233933-7-maxime@cerno.tech