power: supply: axp20x_ac_power: Allow offlining

AXP803/AXP813 have a flag that enables/disables the AC power supply
input. Allow control of this flag via the ONLINE property on those
variants.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>