power: supply: axp20x_usb_power: Allow offlining

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

It may be necessary to offline the USB power supply input when using
the USB port in OTG mode, or to allow userspace to disable charging.

When the USB VBUS input is disabled via the PATH_SEL bit, the VBUS_USED
bit in PWR_INPUT_STATUS is cleared, so there is no change needed when
getting the property.

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>