dt-bindings: Add AXP803's regulator info

AXP803 have the most regulators in currently supported AXP PMICs.

Add info for the regulators in the dt-bindings document.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>