usb: otg: gpio_vbus: Add otg transceiver events and notifiers

Commit 9ad63986c606 (pda_power: Add support for using otg transceiver events)
converted the pda-power driver to use otg events to determine the status
of the power supply.

As gpio-vbus didn't use otg events until now, this change breaks setups
of pda-power with a gpio-vbus transceiver.

This patch adds the necessary otg events and notifiers to gpio-vbus.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Dima Zavin <dima@android.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>