AXP803 have the most regulators in currently supported AXP PMICs.

Add info for the regulators in the dt-bindings document.
Signed-off-by: NIcenowy Zheng <icenowy@aosc.io>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>