ARM: dts: sun8i-a33: sina33: Add missing panel power supply

The SinA33 panel is missing its power-supply property, even though the
binding mandates it.

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/20210114113538.1233933-15-maxime@cerno.tech