ARM: dts: sun8i: nanopi-duo2: Fix GPIO regulator state array

Even though it translates to the same thing down to the binary level, we
should have an array of 2 number cells to describe each voltage state,
which in turns create a validation warning.

Let's fix this.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>