ARM: dts: sun9i: cubieboard4: Add AXP809 PMIC device node and regulators

The AXP809 PMIC is the primary PMIC. It provides various supply voltages
for the SoC and other peripherals. The PMIC's interrupt line is connected
to NMI pin of the SoC.

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