ARM: dts: sun9i: cubieboard4: Declare AXP809 SW regulator as unused

The AXP809's SW (switch) regulator is unused on the Cubieboard 4.
Add an empty node for it so that the OS can generate constraints.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>