usb: gadget: udc: atmel: implement .pullup callback

Implement udc->pullup callback, so that udc_connect/disconnect work.
This is needed for composite gadget, as it assumes udc_disconnect()
actually works and calls its ->disconnect callback.

Acked-by: Cristian Birsan <cristian.birsan@microchip.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Felipe Balbi <balbi@kernel.org>