ARM: dts: sun9i: cubieboard4: Add AXP806 PMIC device node and regulators

The AXP806 PMIC is the secondary 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>