arm64: allwinner: a64: add AXP803 PMIC support to SoPine and the baseboard

The SoPine SoM has an AXP803 PMIC connected to the RSB bus of the A64
SoC, and the regulators of the PMIC are used both on the SoM itself and
on the official baseboard

Add related device tree parts to the SoPine SoM DTSI file and the
baseboard DT.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>