usb: musb: ux500: add otg notifier support

Add transceiver notifier event handling to the ux500 driver to set vbus
on specific transceiver events.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>

[ balbi@ti.com: fix build error due to missing
	otg_state_string() ]

Signed-off-by: Felipe Balbi <balbi@ti.com>