USB: pxa27x_udc: add vbus session handling

On vbus_session() call, optionally activate D+ pullup
resistor and enable the udc, or deactivate D+ pullup
resistor and disable the udc.

It is intentional to not handle any VBus sense related irq.
An external transceiver driver (like gpio_vbus) should
catch VBus sense signal, and call usb_gadget_vbus_connect()
or usb_gadget_vbus_disconnect().

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>