MLK-10132-3 usb: chipidea: udc: do not enter low power mode if vbus on

MLK-10132-3 usb: chipidea: udc: do not enter low power mode if vbus on

This patch is to prevent usb entering low power mode if vbus is on even gadget
driver is not binded, by holding the PM count of ci->dev.
So, there are 3 pm usage_count status:
- ci->dev: 1 ci->gadget.dev: 1
  Device mode with gadget driver binded and vbus on.
- ci->dev: 1 ci->gadget.dev: 0
  USB vbus on but gadget driver not binded.
- ci->dev: 0 ci->gadget.dev: 1
  USB OTG FSM is in a_peripheral mode.
Above 2 device's pm usage_count hold by ci otg(ci->dev) and usb gadget
(ci->gadget.dev).

Signed-off-by: Li Jun <jun.li@freescale.com>