usb: common: usb-conn-gpio: Register charger

usb: common: usb-conn-gpio: Register charger

Register a power supply charger, whose online state depends on whether
the USB role is set to device or not.

This is useful when the USB role is the only way to know if the device
is charging from USB. The API is the standard power supply charger API,
you get a /sys/class/power_supply/xxx/online node which tells you the
state of the charger.

The sole purpose of this is to give userspace applications a way to
know whether or not the charger is plugged.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20200727170413.23131-1-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>