arm64: allwinner: a64: add DTSI file for AXP803 PMIC

As nearly all A64 boards are using AXP803 PMIC, add a DTSI file for it,
like the old DTSI files for AXP20x/22x, for the common parts of the
PMIC.

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