regulator: axp20x-regulator: add support for AXP803

AXP803 PMIC also have a series of regulators (DCDCs and LDOs)
controllable via I2C/RSB bus.

Add support for them.

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