Doc: ABI: add usb charger uevent

When the USB charger is inserted or removed, the users could get
USB charger state and type through the uevent.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Link: https://lore.kernel.org/r/1579145333-1657-2-git-send-email-peter.chen@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>