arm64: allwinner: a64: add AXP803 node to Pine64 device tree

The Pine64 (including Pine64+) boards have an AXP803 as its main PMIC.

Add its device node.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>