mfd: axp20x: Add axp20x-regulator cell for AXP803

As axp20x-regulator now supports AXP803, add a cell for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>