USB: otg: add notifier support

The notifier will be used to communicate usb events
to other drivers like the charger chip.

This can be used as source of information to kick
usb charger detection as described by the USB
Battery Charging Specification 1.1 and/or to
pass bMaxPower field of selected usb_configuration
to charger chip in order to use that information
as input current on the charging profile
setup.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>